Remove the global step_multi in favour of a per-thread

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

	* inferior.h (step_multi): Delete.
	* gdbthread.h (struct thread_info): Add comments around
	step_multi.
	(save_infrun_state, load_infrun_state): Remove step_multi
	parameter.
	* thread.c (load_infrun_state, save_infrun_state): Remove
	step_multi argument, and references to it.
	* infcmd.c (step_multi): Delete.
	(step_1): Adjust.
	(step_1_continuation, until_next_command): Adjust.
	* infrun.c (fetch_inferior_event): Adjust.
	(context_switch): Don't context-switch step_multi.
	(print_stop_reason, normal_stop): Adjust.