Add a configurable limit on realname length

See #2113 for motivation

Channels with slashes (or other relaymsg separators) in their names
were being falsely detected as relaymsg identifiers.

Persisting always-on clients was panicking if client X believed it was
a member of channel Y, but channel Y didn't have a record of client X.

Remove numerics associated with the retired ACC spec

Send Message-ID even if DKIM is not enabled, for compatibility with Gmail:

* A workaround for Ergo 2.12.0 is to enable DKIM
* You need to enable either DKIM or SPF (preferably both) to send to Gmail anyway
* You also need forward-confirmed reverse DNS, which can be tricky for IPv6...

* Fix #1997 (allow the use of an external file for the email blacklist)
* Change config key names for blacklist (compatibility break)
* Accept globs rather than regexes for blacklist by default
* Blacklist comparison is now case-insensitive

See discussion on #2077

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

This was introduced in 38a6d17ee5

* don't send multiline responses to CAP LS 301

This is more or less explicitly prohibited by the spec:

https://ircv3.net/specs/extensions/capability-negotiation.html#multiline-replies-to-cap-ls-and-cap-list

* switch to whitelist model to be future-proof

* bump irctest to include test

* add a unit test

* Makefile: Add dependencies between targets

* Implement draft/message-redaction for channels

Permission to use REDACT mirrors permission for 'HistServ DELETE'

* Error when the given targetmsg does not exist

* gofmt

* Add CanDelete enum type

* gofmt

* Add support for PMs

* Fix documentation of allow-individual-delete.

* Remove 'TODO: add configurable fallback'

slingamn says it's probably not desirable, and I'm on the fence.
Out of laziness, let's omit it for now, as it's not a regression
compared to '/msg HistServ DELETE'.

* Revert "Makefile: Add dependencies between targets"

This reverts commit 2182b1da69.

---------

Co-authored-by: Val Lorentz <progval+git+ergo@progval.net>

CHATHISTORY TARGETS response should not be in a batch unless the client has
explicitly requested the batch cap.

In #2058 we introduced two bugs:

* A nil dereference when an outside user attempts to speak
* Ordinary copy of a modes.ModeSet (which should only be accessed via atomics)

This fixes both issues.

* tweaks to NAMES implementation

* tweak member caching

* add a benchmark for NAMES

See #2052

Remove `<no reason supplied>`, make default KILL anonymous

Fixes #2048

* 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

https://github.com/ircv3/ircv3-specifications/pull/510

See #2037

Sanitize the in-band error message from REHASH

Fixes #1782

Also stop attaching the *tls.Config to the wrapped connection,
since this forces it to be retained beyond its natural lifetime.