Handling of reserved nicknames is special-cased due to #1594, but we want to send
ERR_NICKNAMEINUSE if the nickname is actually in use, since that doesn't pose any
client compatibility problems.

See #2013; given that plaintext is deprecated now, it seems like there is no
added value from continuing to police this.

* implement draft/pre-away
* clean up some subtleties in auto-away aggregation.
* consistently apply auto-away only to always-on
* `AWAY *` should not produce user-visible changes wherever possible

If the nickname must equal the account name (because always-on or
force-nick-equals-account), the correct error response to an empty
or otherwise invalid nickname is the usual "You must use your account
name as your nickname".

Don't allow any new uses of 0 as a nickname, since it conflicts with
the use of 0 as a placeholder for account name in WHOX.

It collides with the massmessage mask syntax. Reported by @emersion

Implements the new `CHATHISTORY LISTCORRESPONDENTS` API.

Transition most "is an operator" checks to require a specific operator
capability

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.