`WHO #channel o` is supposed to return only server operators.
This is RFC1459 cruft; just return an empty list in this case.

RPL_ENDOFWHO should send the original, un-normalized mask

Ensure the pagination window is full by making sure that every history item
gets a replay line in CHATHISTORY output, even TAGMSG.

This reverts commit 5bbee02fe6.

Fix various pagination issues with CHATHISTORY; also undo #491
(msgid munging).

Fixes #1787

Warn if the user sent, e.g., /QUOTE /SAMODE instead of /QUOTE SAMODE

Refs:

* other instances in the codebase
* https://defs.ircdocs.horse/defs/numerics.html#err-needmoreparams-461
* https://modern.ircdocs.horse/#errneedmoreparams-461

For consistency with RFC2812, Bahamut, Hybrid, Insp, Plexus4, Unreal.
https://datatracker.ietf.org/doc/html/rfc2812#section-3.2.8

At the expense of consistency with chary/solanum, irc2, and ircu2.

reported by @Mikaela

We can check once during initialization of the SASL session, e.g.
on receiving `AUTHENTICATE PLAIN` or `AUTHENTICATE EXTERNAL`

See: https://github.com/ircv3/ircv3-specifications/pull/466

This is one of the two cases of [RFC 2812 kicks](https://datatracker.ietf.org/doc/html/rfc2812#section-3.2.8):
even when there are multiple user targets, the RFC (and Unreal
and Inspircd and probably others) allows a single channel name.

Fixes #1740

See #1659

CHATHISTORY INVALID_TARGETS was missing the subcommand parameter

It could be the real IP or the proxied IP.

`WHOWAS :` should return an error numeric instead of an empty response

If the relay bot and the owner share an IP, legacy bots that identify users
by user@host could misinterpret relayed lines as coming from the bot owner.
Try to avoid this by using the bot's account cloak where applicable.

Send a full NUH with RELAYMSG.

Also fix client-only tags with RELAYMSG.

Remove the CHATHISTORY * and znc.in/playback *self targets,
clean up associated database code, add new mechanisms to play
all missed DMs.

Remove debugging loglines for truncation

Implements the new `CHATHISTORY LISTCORRESPONDENTS` API.

All truncation must be made UTF8-safe.

AWAY status should be tracked per-session:

1. With auto-away enabled, away status is aggregated across sessions
   (if any session is not away, the client is not away, else use
   the away status that was set most recently)
2. With auto-away disabled, we get the legacy behavior where AWAY
   applies directly to the client

Implement the new bot mode spec:
https://github.com/ircv3/ircv3-specifications/pull/439

RELAYMSG should respect mutes

Implements #1577, but the issue should remain open until we clean up
the debugging loglines.

Fixes #1064.

1. Send COMPLETE_CONNECTION_REQUIRED instead of DISALLOWED
2. Include the account name in all FAIL messages

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

Registered channels should be eagerly created on startup, and should
remain (and be visible in LIST) even when they have no members.