Add enum for result of fast_tracepoint_collecting

Add enum for result of fast_tracepoint_collecting

I got confused by the result value of fast_tracepoint_collecting, while
it sounds like it would return true/false (whether the thread is
collecting or not), it actually returns:

  0: not collecting
  1: in the jump pad, before the relocated instruction
  2: in the jump pad, at or after the relocated instruction

To avoid confusion, I think it would be nice to make it return an enum.
If you can help find a shorter but still relavant name, it would be
awesome.  Otherwise, we'll go with that, fast_tpoint_collect_result,
which is at least consistent with the existing
fast_tpoint_collect_status.

gdb/gdbserver/ChangeLog:

	* tracepoint.h (enum class fast_tpoint_collect_result): New
	enumeration.
	(fast_tracepoint_collecting): Change return type to
	fast_tpoint_collect_result.
	* tracepoint.c (fast_tracepoint_collecting): Likewise.
	* linux-low.h: Include tracepoint.h.
	(struct lwp_info) <collecting_fast_tracepoint>: Change type to
	fast_tpoint_collect_result.
	* linux-low.c (handle_tracepoints): Adjust.
	(linux_fast_tracepoint_collecting): Change return type to
	fast_tpoint_collect_result.
	(maybe_move_out_of_jump_pad, linux_wait_for_event_filtered,
	linux_wait_1, stuck_in_jump_pad_callback,
	lwp_signal_can_be_delivered, linux_resume_one_lwp_throw,
	proceed_one_lwp): Adjust to type change.