Add some cleverness for breaking infinite loops.  Previously we'd only
detect it successfully if you connected to exactly the same IP as we were
listening on, but that was unreliable if we're listening on 0.0.0.0 and you
connected to one of the IP addresses we haven't heard of.

Now, if you try to connect to our listen port on *any* IP, we try binding to
that IP as a local socket; if it works, that's a local IP, and therefore
it's our socket, so reject the connection.  If it doesn't work, it's a
remote IP, so forward it along.

Thanks to 'tass' on github for noticing the problem.