Previously, we were applying defaults before the user had completed
registration. This meant that the number of invisible users was
incremented when the user connected, and then the total was incremented
when registration was completed.

Now both counters are updated at the same time. If a user disconnects
prior to registration, +i has not yet been applied so it would not be
decremented.

This avoids "No such nick" responses to +draft/typing TAGMSG, etc.

1. TAGMSG were incorrectly being considered multilines, because
   Is512() was checking the wrong thing
2. Playback of TAGMSG should depend on event-playback, not on message-tags

don't need to send account-notify * (logout notification)
because either we never sent the initial account-notify
(if we are being logged out as part of fixupNickEqualsAccount)
or else we are immediately going to kill the client
(if we are being logged out as part of unregister/erase)