src/stacktrace.c: Add an option to print build-ids.

src/stacktrace.c: Add an option to print build-ids.

In preparation for adapting eu-stacktrace to gmon.out generation,
print out per-frame build-ids to sanitycheck the data.

* src/stacktrace.c (show_buildids): New global variable.
  (struct sysprof_unwind_info): Enable sui->last_dwfl.
  (sysprof_unwind_frame_cb): Obtain and print build-id.
  (sysprof_unwind_cb): Enable sui->last_dwfl.
  (parse_opt): Accept --buildid option.
  (main): Accept --buildid option and env var setting.