libdwfl_stacktrace [9/12]: add dwflst_tracker_find_pid

libdwfl_stacktrace [9/12]: add dwflst_tracker_find_pid

Changes for v6:

- Minor fixes as requested.

Changes for v4:

- Separate out libdwfl_stacktrace, as requested.

Changes for v2:

- Add locking for dwfltab.

* * *

New function that retrieves the Dwfl for a particular PID, or,
if the Dwfl is absent, creates it via a provided callback
and adds it to the table later, when the PID is confirmed
via dwfl_attach_state.

* libdwfl_stacktrace/libdwfl_stacktrace.h (dwflst_tracker_find_pid):
  New function.
* libdwfl_stacktrace/dwfl_process_tracker.c (dwflst_tracker_find_pid):
  New function; find a Dwfl in the dwfltab or create one using the
  provided callback.  The newly created Dwfl will be added to the
  dwfltab automatically when its pid is confirmed by a call to
  dwfl_attach_state.
* libdw/libdw.map: Add dwflst_tracker_find_pid.