MEDIUM: protocol: do not call proto->bind() anymore from bind_listener()

MEDIUM: protocol: do not call proto->bind() anymore from bind_listener()

All protocol's listeners now only take care of themselves and not of
the receiver anymore since that's already being done in proto_bind_all().
Now it finally becomes obvious that UDP doesn't need a listener, as the
only thing it does is to set the listener's state to LI_LISTEN!