Implement -trace-find.

	* mi/mi-cmds.c (mi_cmds): Register -trace-find.
	* mi/mi-cmds.h (mi_cmd_trace_find): Declare.
	* mi/mi-main.c (mi_cmd_trace_find): New.
	* target.h (struct target_ops): Document to_trace_find.
	* tracepoint.h (tfind_1): Declare.
	* tracepoint.c (finish_tfind_command): Rename to...
	(tfind_1): ...this.
	* remote.c (remote_trace_find): Return -1 if target say
	there's no frame.  Improve error diagnostics.