Added --continue and --only-clone. Improved lysrdiff-monitord.

New options --continue and --only-clone.  Better remote watching of
the status.

* backup-one-task: Handle --only-clone.  Adjusted to the new CLI
  of lysrdiff-set-status.

* lysrdiff-set-status.py
  (open_socket): New function.
  (sendmsg): Use it to simplify code.
  (completion): New function.
  (Top level): New mandatory first argument: --status, --ok, --warning
    or --fail.

* backup-all: Added --continue and --only-clone.

* lysrdiff-monitord.py
  (now): New function.
  (Client.__init__): New API: use sock and peer arguments instead of
    accept_retval.  Renamed the input buffer from __readbuf to
    _readbuf so that derived classes can use it.
  (Client.read_event): Fixed misnamed errno name.  Track name change
    of _readbuf.
  (Client.got_eof): Track name change of _readbuf.
  (Client.write_event): Handle EAGAIN, ETIMEDOUT and EHOSTUNREACH errors.
  (Client.parse): Track name change of _readbuf.
  (Client.handle_cmd): New command: completed.
  (Client.dispatcher): New method.
  (Server.read_event): Track API change of Client.  Set the new client
    socket nonblocking.
  (Vt100Client.__init__): Negotiate telnet to get a raw byte channel
    using WILL ECHO, WILL SuppressGoahead, DO SuppressGoahead.  Use
    vt100 escape sequences to disable line wrap and query the screen
    size.  Don't register with the dispatcher.
  (Vt100Client.parse): New method.  Register with the dispatcher once
    the screen size report has arrived from the client.
  (Dispatcher.toploop): Write the curren time to vt100 clients every
    second.
  (Dispatcher.register_vt100): Position the cursor at the command line
    position.
  (Dispatcher.write_monitor_status): Format the status separately for
    each client, as their screen sizes may vary.
  (Dispatcher.format_monitor_status): New argument: client.
    Position the cursor properly.
  (Dispatcher.set_status): Use now() to simplify code.  Don't crash if
    a client connection is found to be broken.  Format the status
    separately for each client, as their screen sizes may vary.
  (Dispatcher.format_status_vt100): New argument: client.  Truncate
    the status to the line length reported by the client.  Position
    the cursor at the command line position.
  (Dispatcher.completed): New method.