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)

This avoids conditions where two sessions can be attached to the same client
while not logged into an account.

This adds a new configuration option, socket-path, instead of using
host.

This ensures that counts like the number of invisible users are updated
correctly.

This affects /TOPIC, /INVITE, and a few others.

As of 3dc5c8de78, the PROXY-before-TLS proxy line
and any proxy information carried in X-Forwarded-For
do not count as the "first line", so an additional
client-supplied PROXY line would have been accepted.

Construct a hash set of the user's channels and check that rather than
querying channel membership, to reduce the number of locks that need to be
acquired.

Fixes #990.