publish: Add '--listen'.

* guix/scripts/publish.scm (show-help, %options): Add --listen.
  (getaddrinfo*): New procedure.
  (%default-options): Add 'address'.
  (open-server-socket): Replace 'addr' and 'port' with 'address', a
  sockaddr.
  (guix-publish): Adjust accordingly.  Augment "publishing" message with
  the actual address.
* doc/guix.texi (Invoking guix publish): Document it.