Fix a few fd leaks

/sys/kernel/debug/tracing.

add --kernel-threads-only to tools/offcputime

Summary:
Adds `--kernel-threads-only` arg
The kernel-threads-only arg is exclusive with pid/user-threads-only via `parser.add_mutually_exclusive_group`.
The output message now indicates what we are tracing (pid/user threads/kernel threads/all threads).
Removed the --verbose arg (unused).

Test Plan:
Run with combinations of the args; validate output looks sane:

// test mutually exclusive group
```
devbig680[bcc](abirchall_dev): ~/bcc_run_tool.sh offcputime -k -u 1
[Running] /data/users/abirchall/bcc/tools/offcputime.py -k -u 1
usage: offcputime.py [-h] [-p PID | -k | -u] [-v] [-f] [duration]
offcputime.py: error: argument -u/--user-threads-only: not allowed with argument -k/--kernel-threads-only
```

// kernel threads only
```
devbig680[bcc](abirchall_dev): ~/bcc_run_tool.sh offcputime -f -k 1
[Running] /data/users/abirchall/bcc/tools/offcputime.py -f -k 1
swapper/21;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 11
swapper/16;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 19
swapper/22;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 20
swapper/31;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 20
swapper/23;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 67
swapper/25;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 74
...
```
`~/bcc_run_tool.sh offcputime -f --kernel-threads-only 1`

// user threads only
`~/bcc_run_tool.sh offcputime -f --user-threads-only 1`
`~/bcc_run_tool.sh offcputime -f -u 1`

// specific pid
`~/bcc_run_tool.sh offcputime -f -p $(pidof hphpi) 1`
```
devbig680[bcc](abirchall_dev): ~/bcc_run_tool.sh offcputime --pid $(pidof mcrouter) 10 | head
[Running] /data/users/abirchall/bcc/tools/offcputime.py --pid 95929 10
Tracing off-CPU time (us) of PID 95929 by kernel stack for 10 secs.
```

Note that this last case (specific PID) doesn't appear to be working; I can debug that in a follow up commit.

handle ENOMEM in tools/offcputime

Summary:
* move ENOMEM comment from tools/offcputime.py to src/cc/export/helpers.h
* create --stack-storage-size arg to allow the user to set the stack storage size
    * requires a positive_nonzero_int type checking function for argparse
* clean up the rest of the args a bit
    * use parser.add_mutually_exclusive_group instead of manually handling the
      mutual exclusion
    * use positive_nonzero_int for duration arg
    * use positive_int for pid arg
* only print a warning about increasing the storage size if at least one of the
  get_stackid errors was a -ENOMEM
* remove the debug param (we can add this manually when testing)
* fix a bug where all processes are traced when specifying pid of 0

Test Plan:
```
devbig680[bcc](no branch, rebasing abirchall_dev): ~/bcc_run_tool.sh offcputime --help
[Running] /data/users/abirchall/bcc/tools/offcputime.py --help
usage: offcputime.py [-h] [-u | -p PID] [-v] [-f]
                     [--stack-storage-size STACK_STORAGE_SIZE]
                     [duration]

Summarize off-CPU time by kernel stack trace

positional arguments:
  duration              duration of trace, in seconds

optional arguments:
  -h, --help            show this help message and exit
  -u, --useronly        user threads only (no kernel threads)
  -p PID, --pid PID     trace this PID only
  -v, --verbose         show raw addresses
  -f, --folded          output folded format
  --stack-storage-size STACK_STORAGE_SIZE
                        the number of unique stack traces that can be stored
                        and displayed

examples:
    ./offcputime             # trace off-CPU stack time until Ctrl-C
    ./offcputime 5           # trace for 5 seconds only
    ./offcputime -f 5        # 5 seconds, and output in folded format
    ./offcputime -u          # don't include kernel threads (user only)
    ./offcputime -p 185      # trace fo PID 185 only
devbig680[bcc](no branch, rebasing abirchall_dev): ~/bcc_run_tool.sh offcputime -f 0
[Running] /data/users/abirchall/bcc/tools/offcputime.py -f 0
usage: offcputime.py [-h] [-u | -p PID] [-v] [-f]
                     [--stack-storage-size STACK_STORAGE_SIZE]
                     [duration]
offcputime.py: error: argument duration: must be positive and nonzero
devbig680[bcc](no branch, rebasing abirchall_dev): ~/bcc_run_tool.sh offcputime -f -1
[Running] /data/users/abirchall/bcc/tools/offcputime.py -f -1
usage: offcputime.py [-h] [-u | -p PID] [-v] [-f]
                     [--stack-storage-size STACK_STORAGE_SIZE]
                     [duration]
offcputime.py: error: argument duration: must be positive and nonzero
devbig680[bcc](no branch, rebasing abirchall_dev): ~/bcc_run_tool.sh offcputime --stack-storage-size 0 -f 1
[Running] /data/users/abirchall/bcc/tools/offcputime.py --stack-storage-size 0 -f 1
usage: offcputime.py [-h] [-u | -p PID] [-v] [-f]
                     [--stack-storage-size STACK_STORAGE_SIZE]
                     [duration]
offcputime.py: error: argument --stack-storage-size: must be positive and nonzero
devbig680[bcc](no branch, rebasing abirchall_dev): ~/bcc_run_tool.sh offcputime --stack-storage-size 1 -f 1
[Running] /data/users/abirchall/bcc/tools/offcputime.py --stack-storage-size 1 -f 1
swapper/27;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 7
swapper/29;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 102
swapper/25;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 113
swapper/26;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 138
swapper/28;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 164
swapper/30;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 177
swapper/24;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 980
swapper/31;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 1556
swapper/1;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 2038
swapper/3;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 2190
swapper/2;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 2235
swapper/15;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 2260
swapper/13;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 2450
swapper/6;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 2646
swapper/12;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 2675
swapper/11;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 2769
swapper/8;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 2885
swapper/4;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 3134
swapper/10;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 3416
swapper/14;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 3702
swapper/7;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 4343
swapper/9;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 5397
swapper/5;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 6836
swapper/17;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 13792
swapper/19;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 14488
swapper/22;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 15298
swapper/18;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 15735
swapper/20;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 17333
swapper/16;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 17645
swapper/23;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 17681
swapper/21;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 18607
WARNING: 92 stack traces could not be displayed. Consider increasing --stack-storage-size.

Summary:
BPF_STACK_TRACE(_name, _size) will allocate space for _size stack traces
(see https://github.com/torvalds/linux/blob/master/kernel/bpf/stackmap.c#L30-L50).

If we've already used all of this space, subsequent calls to bpf_get_stackid()
will return -ENOMEM (see https://github.com/torvalds/linux/blob/master/kernel/bpf/stackmap.c#L173-L176).

This causes our BPF bytecode to store this value in key_t.stack_id and
subsequently causes our python application to crash due to a KeyError when
invoking stack_traces.walk(k.stack_id).

Let's avoid calling stack_traces.walk(k.stack_id) with back stackid's

Test Plan:
Run offcputime.py in an extreme case; with space for only a single stack trace
```
devbig680[bcc](tools): sed_in_file 's/BPF_STACK_TRACE(stack_traces, 1024)/BPF_STACK_TRACE(stack_traces, 1)/' tools/offcputime.py && \
> ~/bcc_run_tool.sh offcputime -f 5; \
> git reset --hard HEAD
swapper/30;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 496
swapper/26;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 553
swapper/28;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 604
swapper/31;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 692
swapper/23;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 713
swapper/18;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 919
swapper/16;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 1051
swapper/20;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 1056
swapper/21;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 1585
swapper/24;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 1597
swapper/27;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 1610
swapper/17;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 1674
swapper/22;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 2390
swapper/25;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 2574
swapper/19;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 2589
swapper/29;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 8428
swapper/8;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 15272
swapper/15;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 15591
swapper/11;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 17934
swapper/9;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 18100
swapper/14;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 18266
swapper/10;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 20124
swapper/12;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 20887
swapper/13;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 23453
swapper/3;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 27296
swapper/5;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 29094
swapper/6;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 29799
swapper/7;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 31522
swapper/1;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 32269
swapper/4;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 35585
swapper/2;start_secondary;cpu_startup_entry;schedule_preempt_disabled;schedule 37627
WARNING: 249 stack traces could not be displayed. Consider increasing stack trace storage size.
HEAD is now at d3365e9 [RFC] handle ENOMEM in tools/offcputime`
```

http_filter fixes

Lua ports of biosnoop and stacksnoop

updated cmake system to include lua and new networking example

Add new example with test case for for unroll, as requested.

Revert "Force lua main.c to include luajit headers"

This reverts commit c21b9c03.

Instead, use `LUAJIT_DIR=/usr/include/luajit-2.0 cmake ..` or similar as
required.
Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>

Add dns_matching example with test case for for unroll, as requested at iovisor-dev@lists.iovisor.org

Lua installation instructions.

[RFC] New USDT API

Add check for number of arguments

sys.exc_value and sys.exc_type should come from sys.exc_info()

Convert (decode) char* return values from C functions to a native python
str.
Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>

There are two problems here:
 1. bcc is leaving the extra function parameters in the prototype, when
 these really just serve as an annotation on the method being traced.
 They aren't really passed into the function. LLVM lowering phase later
 errors out on such functions.
 2. bcc doesn't limit the size of the argument list, when currently it
 just supports up to the number of arguments held in registers (6).

Fix 1. by rewriting the arguments out of the prototype and into the
preamble where they are currently being initialized.
Fix 2. by enforcing the limit and returning a more meaningful error
message.

Added a test case.

Extra fix included for string type on python3.

Fixes: #497
Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>

Introduce helpers to access pt_regs in an arch-independent manner

Convert some of the examples and tools to use the new helpers.
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>

fix llvm 3.9 build

Signed-off-by: Brenden Blanco <bblanco@plumgrid.com>

Force lua main.c to include luajit headers