basics for a hypothetical PortRename callback

This cannot be implemented at present because jack_port_set_name() does not take
a jack_client_t* as an argument, and thus no msg can be sent to the server
regarding the name change. Jack2 accomplishes this by walking its static array
of clients to find one that is in use and sends the msg using that one. This
is not possible in Jack1 because we do not use static arrays of clients (or
ports).