eu-stacktrace [7/12]: use Dwflst_Process_Tracker for Elf * caching

eu-stacktrace [7/12]: use Dwflst_Process_Tracker for Elf * caching

Changes for v4:

- Separate out libdwfl_stacktrace, as requested.

* * *

* src/Makefile.am (AM_CPPFLAGS): Include headers from
  ../libdwfl_stacktrace.
* src/stacktrace.c (tracker): New global variable.
  (sample_callbacks): Use dwflst_tracker_linux_proc_find_elf for
  caching.
  (sysprof_init_dwfl): Use dwflst_tracker_dwfl_begin.
  (main): Initialize and clean up tracker.