Per-process displaced stepping queue.

	* infrun.c (displaced_step_ptid, displaced_step_request_queue)
	(displaced_step_gdbarch, displaced_step_closure,
	(displaced_step_original, displaced_step_copy): Move globals to
	this...
	(struct displaced_step_inferior_state): ... new structure.
	(displaced_step_inferior_states): New global.
	(get_displaced_stepping_state, add_displaced_stepping_state)
	(remove_displaced_stepping_state, infrun_inferior_exit): New
	functions.
	(displaced_step_clear): Add displaced_step_inferior_state
	parameter, and adjust to handle it.
	(displaced_step_clear_cleanup): Parameter is now a
	displaced_step_inferior_state.  Adjust.
	(displaced_step_prepare): Adjust.
	(displaced_step_fixup, displaced_step_fixup)
	(infrun_thread_ptid_changed, resume): Adjust.
	(init_wait_for_inferior): Don't call displaced_step_clear.
	(infrun_thread_stop_requested): Rewrite.
	(_initialize_infrun): Install infrun_inferior_exit as
	inferior_exit observer.