Add new `knock' protocol.

Add new `knock' protocol.

  * Add a new option to the `ADD' command giving the knock string to
    send.

  * Add new protocol messages `token-rq', `token', and `knock', which
    together allow a possibly unknown peer to collect a single-use
    token (`token-rq' and `token') which it can use to identify itself,
    and use this to introduce itself to its (presumably) static
    peer (`knock').  This last message can either provoke a `KNOCK'
    notification to a service, or initiate key-exchange using a
    piggybacked `pre-challenge', possibly after having updated the
    peer's address.

  * Update the `connect' service to collect the necessary knock strings
    from the peer database, and to respond to `KNOCK' notifications.

  * Update the Wireshark dissector to recognize and dissect the new
    messages.  (The `cap.knock' file sneakily includes a message from
    the future, to be dissected in a later modification.)