[PATCH] Added Null packet keepalive option

[Markus Friedl]

SSH_MSG_NONE is no message that is allowd to appear on
the wire, it's not defined at all.  SSH_MSG_IGNORE breaks
ssh-1.2.18 to 1.2.22, but i think they'll accept an ignore message
w/o payload.

> +                /* Send a keepalive packet (not SSH_MSG_IGNORE as this crashes
> +                 * some servers...).
> +                 */
> +                packet_start(SSH_MSG_NONE);
> +                packet_send();
> +                break;