C++fy thread_apply_all_command

C++fy thread_apply_all_command

This eliminates a couple cleanups.

gdb/ChangeLog:
2017-04-13  Pedro Alves  <palves@redhat.com>

	* thread.c: Include <algorithm>.
	(thread_array_cleanup): Delete.
	(scoped_inc_dec_ref): New class.
	(live_threads_count): New function.
	(set_thread_refcount): Delete.
	(tp_array_compar_ascending): Now a bool.
	(tp_array_compar): Convert to a std::sort comparison function.
	(thread_apply_all_command): Use std::vector and scoped_inc_dec_ref
	and live_threads_count.