MINOR: listener: automatically set the port when creating listeners

MINOR: listener: automatically set the port when creating listeners

In create_listeners() we iterate over a port range and call the
protocol's ->add() function to add a new listener on the specified
port. Only tcp4/tcp6/udp4/udp6 support a port, the other ones ignore
it. Now that we can rely on the address family to properly set the
port, better do it this way directly from create_listeners() and
remove the family-specific case from the protocol layer.