commot/protocol.scm: Improve the protocol.

Improve the protocol.  Standartize message exchange procedures,
add the <message> class.
* common/message.scm: New file.
* common/protocol.scm: Improve.  Use ABNF notation instead of BNF
  to describe the protocol.
* lazycat-daemon/lazycatd.scm: Improve.  Use <message> class.
* ui/lc.scm: Likewise.
* ui/cli/pretty-format.scm (format-output-list): Remove the extra
  status check.
  (format-diff): Likewise.  Improve.
  * proxies/ssh-proxy/ssh-proxy.scm (handle-set-option!)
  (handle-ping): Change return value.
lazycat-daemon/host-list.scm: Fix a bug.

* lazycat-daemon/host-list.scm (host-list-add-host): Fix a bug:
  increment host ID.
Make lazy-gtk-cat work with the new protocol format.

* ui/gtk/lazy-gtk-cat.scm: Improve.  Use <message> class.
* ui/gtk/output-view.scm (output-view-format-list)
  (output-view-format-diff): Remove extra status checks.  Improve.