Fix meshlink_stop() on Mac OS X.

Fix meshlink_stop() on Mac OS X.

Mac OS X doesn't like shutdown(SHUT_RDWR) on listening sockets, and also
doesn't allow reopening a listening socket right after closing the
previous one on the same port. So avoid the whole business of closing
and reopening sockets by stopping the event loop and then doing a dummy
connect() to ourself to ensure select() in the network thread returns
immediately.

# Conflicts:
#	src/meshlink.c