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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.