pprof: made --show-addresses work
pprof: added support for dumping stacks in --text mode

Which is very useful for diagnosing stack capturing and processing
bugs.
cpuprofiler: disable capturing stacktrace from signal's ucontext

This was reported to cause problems due to libunwind occasionally
returning top level pc that is 1 smaller than real pc which causes
problems.
cpuprofiler: better explain deduplication of top stacktrace entry
pprof: eliminate duplicate top frames if dropping signal frames

In cpu profiles that had parts of signal handler we could have
situation like that:

* PC
* signal handler frame
* PC

Specifically when capturing stacktraces via libunwind.

For such stacktraces pprof used to draw self-cycle in functions
confusing everybody. Given that me might have a number of such
profiles in the wild it makes sense to treat that duplicate PC issue.
cpuprofiler: drop correct number of signal handler frames

We actually have 3 and not 2 of them.
pprof: allow disabling auto-removal of "constant 2nd frame"

"constand 2nd frame" feature is supposed to detect and workaround
incorrect cpu profile stack captures where parts of or whole cpu
profiling signal handler frames are not skipped.

I've seen programs where this feature incorrectly removes non-signal
frames.

Plus it actually hides bugs in stacktrace capturing which we want be
able to spot.

There is now --no-auto-signal-frm option for disabling it.