This adds on-behalf-of="#id" to <listen-socket>. It allows a socket
to handle the traffic that was originally meant of another (virtual)
listen socket.
This moves all the listen socket code into a nice and abstracting file.
Notes:
* Altering listen socket setup does not yet work on config reload.
(Did it ever work?)
* Server will start with no listen sockets. (There are unconfirmed
rumours it sometimes(?) did before.) This is to be re-implemented
in another commit. It can also be improved to work allow checking
on reload or other config changes.
* For slave connections the server address is now checked against
the allow/deny-IP list.
