btrace: async support

btrace: async support

Add support for async command execution.  This fixes the gdb.btrace tests.

	* record-btrace.c: Include event-loop.h and inf-loop.h.
	(record_btrace_resume_exec_dir)
	(record_btrace_async_inferior_event_handler)
	(record_btrace_handle_async_inferior_event): New.
	(record_btrace_open): Create async event handler.
	(record_btrace_close): Delete async event handler.
	(record_btrace_resume): Set record_btrace_resume_exec_dir,
	Mark async event handler.
	(record_btrace_execution_direction): New.
	(init_record_btrace_ops): Initialize to_execution_direction.