Implement -trace-save.

	* mi-cmds.h (mi_cmds_trace_save): Declare.
	* mi-cmds.c (mi_cmds): Register -trace-save.
	* mi/mi-main.c (mi_cmd_trace_save): New.
	* remote.c (remote_save_trace_data): Take const parameter.
	* target.h (struct target_ops::to_save_trace_data): Take
	const parameter.
	* target.c (update_current_target): Adjust to the above.
	* tracepoint.c (trave_save): New, extracted from
	(trace_save_command): ...this.
	(tfile_trace_find): Remove message that is unnecessary now
	that 'tfind' reports found frame.
	* tracepoint.h (trace_save): Declare.