eu-stacktrace [10/12]: use dwflst_tracker_find_pid

eu-stacktrace [10/12]: use dwflst_tracker_find_pid

Changes for v6:

- Minor fixes as requested.

Changes for v4:

- Separate out libdwfl_stacktrace, as requested.

* * *

Initial minimal change to ensure dwflst_tracker_find_pid is
tested. For now, we keep the additional dwfltab implementation in
stacktrace.c, since it's being used to track statistics.

In future follow-ups, it will be good to switch to storing
eu-stacktrace statistics e.g. in dwfl->process->callbacks_arg. This
requires some additional design to keep the statistics from being lost
when a pid is reused and the corresponding processtracker table entry
is replaced.

* src/stacktrace.c (sysprof_init_dwfl): New function.
  (sysprof_find_dwfl): Rename the existing sysprof_init_dwfl.
  Also use dwflst_tracker_find_pid with callback.
  (sysprof_unwind_cb): Rename the existing sysprof_init_dwfl.