darwin-nat: avoid crash while debugging gdb.

it is possible that gdb gets mach exceptions from an unknown inferior.  This
happens when an inferior creates a child and that child gets a signal.

So instead of reporting messages with unknown origins, simply reply to these
notifications. The kernel will then post the unix signal.

gdb/
	* darwin-nat.c (darwin_encode_reply): Add prototype.
	(darwin_decode_exception_message): Reply to unknown inferiors.
	(darwin_decode_message): Handle message by id.  Ignore message
	to unknown inferior.
	(darwin_wait): Discard unknown messages, add debug trace.