Remove the global stop_step in favour of a per-thread

	Remove the global stop_step in favour of a per-thread
	stop_step.

	* inferior.h (stop_step): Delete.

	* gdbthread.h (struct thread_info): Add comments to stop_step.
	(save_infrun_state, load_infrun_state): Remove stop_step argument.
	* thread.c (load_infrun_state, save_infrun_state): Remove
	stop_step argument, and references to it.

	* infrun.c (clear_proceed_status): Clear stop_step.
	(fetch_inferior_event): Adjust.
	(context_switch): Don't context-switch stop_step.
	(handle_inferior_event): Adjust.
	(normal_stop): Adjust.
	(save_inferior_status, restore_inferior_status): Adjust.

	* infcmd.c (stop_step): Delete.
	(step_1, step_1_continuation, step_once, until_next_command):
	Adjust.