MINOR: listener: now use a generic add_listener() function

MINOR: listener: now use a generic add_listener() function

With the removal of the family-specific port setting, all protocol had
exactly the same implementation of ->add(). A generic one was created
with the name "default_add_listener" so that all other ones can now be
removed. The API was slightly adjusted so that the protocol and the
listener are passed instead of the listener and the port.

Note that all protocols continue to provide this ->add() method instead
of routinely calling default_add_listener() from create_listeners(). This
makes sure that any non-standard protocol will still be able to intercept
the listener addition if needed.

This could be backported to 2.3 along with the few previous patches on
listners as a pure code cleanup.