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

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.

unregistering an always-on client would produce
"attempting to persist logged-out client : x"
because the client was always-on, but also being ejected

Explicit quit and ping timeout behave the same way,
but reattach after abandoning/losing the previous session
(without the break being detected server-side) is more aggressive
about replaying missed messages, at the cost of potential duplication.

Or try to, anyway

For irctest compatibility.

Fixes #775

This reverts commit a120cc2443.

In the previous commit, the client would receive a failure message but would
actually remain logged in after an authzid/authcid mismatch. This was a
correctness rather than a security issue, but now it's fixed so that the client
never logs in in the first place.

* Fix #684
* Fix #683
* Add `CHANSERV CLEAR`
* Allow mode changes from channel founders even when they aren't joined
* Operators with the chanreg capability are exempt from max-channels-per-account
* Small fixes and cleanup