libwebsockets

ss: formalize user cb retcodes

It's not safe to destroy objects inside a callback from a parent that
still has references to the object.

Formalize what the user code can indicate by its return code from the
callback functions and provide the implementations at the parents.

 - LWSSSSRET_OK:            no action, OK
 - LWSSSSRET_DISCONNECT_ME: disconnect the underlying connection
 - LWSSSSRET_DESTROY_ME:    destroy the ss object
 - LWSSSSRET_TX_DONT_SEND:  for tx, give up the tx opportunity since nothing to send