A default channel mode of +i would block channel creation; fix this by treating
initial joins as SAJOINs.

Note that it's nontrivial to detect initial join in (*Channel).Join, because
having 0 members does not necessarily indicate a new channel.

Allow SAJOIN or implicit always-on join to override channels.operator-only-creation

RENAME (channel rename) that was a simple case change (e.g.
renaming #chan to #CHAN) would delete the channel :-|

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

Fix error message for privileged NS PASSWD on a nonexistent account

May help clarify TLS configuration issues, possibly others too

Fixes #1733

CHATHISTORY INVALID_TARGETS was missing the subcommand parameter

Validate operator vhosts against the configured (or default)
vhosts.valid-regexp

Fix a panic if the operator class title is empty

It could be the real IP or the proxied IP.

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

Co-authored-by: Mikaela Suomalainen <mikaela@mikaela.info>

fix help for +f mode

Commit 7ce396931c introduced the ability
that every user with an account in the AMODE list of a channel can use
the OP command to restore their modes. Update the chanserv help message
accordingly.

Users logged into an account can see their own email.
Opers with the accreg capability can see all users.

Sort snomasks for display rather than displaying them in hash order

This is a regression introduced in 0d05ab4ff4; playback for an individual
DM target would play all DMs.

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.

Fix auditorium JOIN lines with zero values for the time and msgid tags.

Configurations with require-sasl and open account registration are valid,
but likely unintended. Show a warning about them.

1. Fix auth bypass in the default configuration with the addition of
   server.password (the REGISTER command was allowed before connection
   registration, allowing unauthenticated users to REGISTER and then
   take advantage of skip-server-password)
2. Caution operators against the use of require-sasl without disabling
   user-initiated account registration. (Such a configuration is still valid
   in the case of a public server that requires everyone to register.)