Update: Rewrote listen socket handling code comepletly.
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.