Remove global continuations in favour of a per-thread

	Remove global continuations in favour of a per-thread
	continuations.

	* gdbthread.h (struct thread_info): Add comments around
	continuations and intermediate_continuations.
	(save_infrun_state, load_infrun_state): Delete continuations and
	intermediate_continuations arguments.
	* infrun.c (fetch_inferior_event): Only call normal_stop if
	stop_soon is NO_STOP_QUIETLY.
	(context_switch): Don't context-switch the continuations.
	* thread.c (clear_thread_inferior_resources): Discard all
	continuations of the thread we're clearing.
	(save_infrun_state, load_infrun_state): Delete continuations and
	intermediate_continuations arguments, and the code referencing
	them.
	* utils.c: Include "gdbthread.h".
	(cmd_continuation, intermediate_continuation): Delete.
	(add_continuation): Add thread_info* argument.  Install the
	continuation on it.
	(restore_thread_cleanup): New.
	(do_all_continuations_ptid, do_all_continuations_thread_callback):
	New.
	(do_all_continuations): Reimplement.
	(discard_all_continuations_thread_callback,
	discard_all_continuations_thread): New.
	(discard_all_continuations): Reimplement.
	(add_intermediate_continuation): Add thread_info* argument.
	Install the continuation on it.
	(do_all_intermediate_continuations_thread_callback)
	(do_all_intermediate_continuations_thread): New.
	(do_all_intermediate_continuations): Reimplement.
	(discard_all_intermediate_continuations_thread_callback): New.
	(discard_all_intermediate_continuations_thread): New.
	(discard_all_intermediate_continuations): Reimplement.

	* breakpoint.c (until_break_command): Install the continuation on
	the current thread.

	* defs.h (cmd_continuation, intermediate_continuation): Delete.
	(struct thread_info): Forward declare.
	(add_continuation, add_intermediate_continuation): Add
	thread_info* argument.
	(do_all_continuations_thread, discard_all_continuations_thread)
	(do_all_intermediate_continuations_thread)
	(discard_all_intermediate_continuations_thread): Declare.
	* inf-loop.c (inferior_event_handler): In non-stop only run
	continuations on the thread that stopped.  In all-stop, run
	continuations on all threads.
	* infcmd.c (step_once, finish_command): Adjust.