This uses an extended protocol message to signal that rcon
authentication might require a username, allowing the client to enable
the username prompt.
Add a forgotten logout on key update
This system can easily be extended by independent authors without
collisions, something the old system with plain increasing integers did
not allow.
Do this by utilizing the previously unused message code `NETMSG_NULL`
which has a value of 0.
This works for engine and game messages, snapshot items and events.