remote: Discover server log-port and publish-port without Avahi.

* src/cuirass/remote.scm (zmq-server-info, zmq-worker-request-info-message):
New procedures.
* src/cuirass/remote-server.scm (%log-port, %publish-port): New parameters.
(read-worker-exp): Handle 'worker-request-info message.
(remote-server): Set the new parameters.
* src/cuirass/remote-worker.scm (start-worker): Rename "server" argument. Send
a 'worker-request-info message to discover the server log port and publish
port.
(remote-worker): Adapt it.