libdwfl_stacktrace [5/12]: introduce Dwflst_Process_Tracker

libdwfl_stacktrace [5/12]: introduce Dwflst_Process_Tracker

New data structure to coordinate caching Elf data among multiple Dwfl
structs attached to different processes. Meant to reduce the overhead
for profilers that use elfutils for unwinding.

The caching is well-justified, as the prior approach (e.g. in
eu-stacktrace, sysprof-live-unwinder) of creating a separate Dwfl per
process was wildly redundant, opening ~hundreds of file descriptors
just for each common library such as libc.so and leaking the data.

Initial patch just introduces the struct, to be filled in by the rest
of the patch series.

* libdwfl_stacktrace/libdwfl_stacktrace.h
  (Dwflst_Process_Tracker): New struct.
  (dwflst_tracker_begin): New function.
  (dwflst_tracker_dwfl_begin): New function.
  (dwflst_tracker_end): New function.
* libdw/libdw.map: Add new functions.
* libdwfl_stacktrace/libdwfl_stacktraceP.h
  (struct Dwflst_Process_Tracker): New struct.
* libdwfl/libdwflP.h (Dwflst_Process_Tracker): Typedef forward decl.
  (struct Dwfl): Add 'tracker' field.
* libdwfl_stacktrace/Makefile.am (libdwfl_stacktrace_a_SOURCES):
  Add dwflst_process_tracker.c.
* libdwfl_stacktrace/dwflst_process_tracker.c: New file.
  (dwflst_tracker_begin): Initialize the tracker.
  (dwflst_tracker_dwfl_begin): Initialize Dwfl * with specified tracker.
  (dwflst_tracker_end): Deallocate the tracker.

Signed-off-by: Serhei Makarov <serhei@serhei.io>