PR threads/20743: Don't attempt to suspend or resume exited threads.

PR threads/20743: Don't attempt to suspend or resume exited threads.

When resuming a native FreeBSD process, ignore exited threads when
suspending/resuming individual threads prior to continuing the process.

gdb/ChangeLog:

	PR threads/20743
	* fbsd-nat.c (resume_one_thread_cb): Remove.
	(resume_all_threads_cb): Remove.
	(fbsd_resume): Use ALL_NON_EXITED_THREADS instead of
	iterate_over_threads.