-trace-start/-trace-end/-trace-status.

	-trace-start/-trace-end/-trace-status.

	* mi/mi-cmds.c (mi_cmds): Register -trace-start, -trace-status
	and -trace-stop.
	* mi/mi-cmds.h (mi_cmd_trace_start, mi_cmd_trace_status)
	(mi_cmd_trace_stop): Declare.
	* mi/mi-main.c (mi_cmd_trace_start, mi_cmd_trace_status)
	(mi_cmd_trace_stop): New.
	* tracepoint.c (start_tracing): New, extracted from...
	(trace_start_command): ...this.
	(trace_status_mi): New.
	* tracepoint.h (struct trace_status): Document
	stopping_tracepoint.
	(start_tracing, stop_tracing, trace_status_mi): Declare.