* stackcount: Support user-space functions

Add support for user-space functions in `stackcount` by taking an additional
`-l` command-line parameter specifying the name of the user-space library.
When a user-space library is specified, `stackcount` attaches to a specific
process and traces a user-space function with user-space stacks only.
Regex support for uprobes (similar to what is available for kprobes) is
not currently provided.

Also add a couple of functions to the `BPF` object for consistency.

* bcc: Support regex in attach_uprobe

attach_kprobe allows a regular expression for the function name,
while attach_uprobe does not. Add support in libccc for enumerating
all the function symbols in a binary, and use that in the BPF module
to attach uprobes according to a regular expression. For example:

```python
bpf = BPF(text="...")
bpf.attach_uprobe(name="c", sym_re=".*write$", fn_name="probe")
```

* python: Support regex in attach_tracepoint

Modify attach_tracepoint to take a regex argument, in which case
it enumerates all tracepoints matching that regex and attaches to
all of them. The logic for enumerating tracepoints should eventually
belong in libccc and be shared across all the tools (tplist, trace
and so on).

* cc: Fix termination condition bug in symbol enumeration

bcc_elf would not terminate the enumeration correctly when the
user-provided callback returned -1 but there were still more
sections remaining in the ELF to be enumerated.

* stackcount: Support uprobes and tracepoints

Refactored stackcount and added support for uprobes and tracepoints,
which also required changes to the BPF module. USDT support still
pending.

* bcc: Refactor symbol listing to use foreach-style

Refactor symbol listing from paging style to foreach-style with a
callback function per-symbol. Even though we're now performing a
callback from C to Python for each symbol, this is preferable to the
paging approach because we need all the symbols in the current use
case.

Also refactored `stackcount` slightly; only missing support for USDT
probes now.

* stackcount: Support per-process displays

For user-space functions, or when requested for kernel-space
functions or tracepoints, group the output by process. Toggled
with the -P switch, off by default (except for user-space).

* Fix rebase issues, print pid only when there is one

* stackcount: Add USDT support

Now, stackcount supports USDT tracepoints in addition to
kernel functions, user functions, and kernel tracepoints.
The format is the same as with the other general-purpose
tools (argdist, trace):

```
stackcount -p $(pidof node) u:node:gc*
stackcount -p 185 u:pthread:pthread_create
```

* stackcount: Update examples and man page

Add examples and man page documentation for kernel
tracepoints, USDT tracepoints, and other features.

* stackcount: Change printing format slightly

When -p is specified, don't print the comm and pid. Also,
when -P is specified for kernel probes (kprobes and
tracepoints), use -1 for symbol resolution so that we
don't try to resolve kernel functions as user symbols.
Finally, print the comm and pid at the end of the stack
output and not at the beginning.