Test plan:
```
 $ ./build/src/bpftrace -h &> /dev/null && echo $?
 0
```

This allows operators to use other tools, like bpftool, to
introspect a running program.
Signed-off-by: David Calavera <david.calavera@gmail.com>

Fix use-after-free in BPFtrace::get_arg_values

Docs

implement review feedback: pass vector by const ref, use reinterpret cast, remove move() of returned string

we were moving as far as constructor arg, but afterwards we were copying to member. this fixes that. https://stackoverflow.com/a/16725030/5257399

implement review feedback: avoid copying strings, prefer make_unique, use standard naming convention for private members

Previously get_arg_values was returning a vector of uint64_t values
that could be passed directly to printf(3). For string values
get_arg_values was returning a pointer to a char*. For some cases it
was attempting to handle freeing the char* memory via a stack
allocated std::vector. Unfortunately, this was stack allocated in
get_arg_values so the char* data would get freed before it was used in
the subsequent call to printf().

In other cases get_arg_values was not freeing char* values and was
leaking memory (probe, stack, and ustack).

get_arg_values() now returns a vector of objects of type IPrintable
instead of uint64_t values. Each object has a method .value() that
returns the uint64_t value usable by printf(). For strings this allows
us to keep around the original std::string until after we've called
printf(), so we don't need to strdup() anymore.

Fixes #194

improve tracepoint not found message

[test] fix runtime-tests cmake settings

Co-Authored-By: Birch-san <Birch-san@users.noreply.github.com>

Co-Authored-By: Birch-san <Birch-san@users.noreply.github.com>

Assigning the result of str() to a map broke after
https://github.com/iovisor/bpftrace/pull/299 landed. This commit fixes
it by making sure our call has a type before the final pass of the
semantic analyzer. Also moved the default strlen to bpftrace.h to make
sure the value is correctly set on unit tests.

allow isra symbols in kprobe wildcards

basic positional paramater support

str() to accept optional length

* Add BPFtrace::extract_func_symbols_from_path function

* Refactor to add find_wildcard_matches overload that takes an istream

* Prepare find_wildcard_matches usage for u(ret)probe types

* Add support for wildcard matches with uprobe and uretprobe types

* Update add_probes_uprobe_wildcard test

* Add test for uprobe wildcard match

* Mention uprobe wildcard in readme

* Add uprobe support to codegen

* Clean up whitespace

* Add TODO comment to remove objdump dependency

Improve build times of codegen tests, fix build-time exhaustion of virtual memory

Bad zero

Search PATH for executables when running commands

This makes it easier for the user to run commands. Specifying abosolute
paths to a binary was kind of a pain in the rear.

gather codegen tests into a single compilation unit, for faster build perf (see https://github.com/iovisor/bpftrace/issues/229)

Better non-root error

Add -c CMD option

This patch adds a command running option to bpftrace. The user can now
run something like:

    ./bpftrace -e '...' -c 'sleep 5'

which is a convenience wrapper around something like:

    sleep 5 & ./bpfrace -e '...' -p `pidof sleep`

`-c` is better because it:
* ensures a tighter tracing range around CMD (ie we trace less of the
system while it is not running CMD)
* makes bpftrace exit (which is convenient) when CMD terminates
    * previously, it was not possible to get a full trace of CMDs
    execution and have bpftrace exit upon CMD termination

Test Plan:
Trivial successful example:
```
$ sudo ./build/src/bpftrace -e 'tracepoint:syscalls:sys_enter_nanosleep
{ printf("%s nanoslept\n", comm); }' -c '/bin/sleep 1'
[sudo] password for dlxu:
chdir(/lib/modules/4.19.8-200.fc28.x86_64/build): No such file or
directory
Attaching 1 probe...
sleep nanoslept
splunkd nanoslept
webrtc_audio_mo nanoslept
gnome-terminal- nanoslept
webrtc_audio_mo nanoslept
gnome-terminal- nanoslept
gnome-terminal- nanoslept
gnome-terminal- nanoslept
gnome-terminal- nanoslept
gnome-terminal- nanoslept
gnome-terminal- nanoslept
gnome-terminal- nanoslept
gnome-terminal- nanoslept
gnome-terminal- nanoslept
gnome-terminal- nanoslept

$
```

Ambigous executable:
```
$ sudo ./build/src/bpftrace -e 'tracepoint:syscalls:sys_enter_nanosleep
{ printf("%s nanoslept\n", comm); }' -c 'sleep 1'
chdir(/lib/modules/4.19.8-200.fc28.x86_64/build): No such file or
directory
Attaching 1 probe...
execve: No such file or directory
Failed to spawn child=sleep 1
splunkd nanoslept

$
```

This closes #253