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.

These commands search the registered nicknames/channels for ones
matching the provided regex, or return the entire list.

Only operators with chanreg (for ChanServ) or accreg (for NickServ)
capabilities can use LIST.

Add default-user-modes configuration to conventional.yaml.
Fix comment on DefaultUserModes.