Fixes #2048

I assumed gorilla validated UTF8 for incoming text messages. In fact, the
documentation states:

>It is the application's responsibility to ensure that text messages
>are valid UTF-8 encoded text.

and this applies to both incoming and outgoing messages. Consequently,
even when enforce-utf8 is enabled, it was possible to send invalid UTF8
to Ergo inside a websocket text frame. This data would be incorrectly
considered valid UTF8, and could be relayed to other clients, including
to websocket clients inside a text frame. The resulting frame would violate
the websocket protocol, causing web clients to be disconnected.

Provide a nondestructive stack trace dump option even when the http pprof
listener is disabled

TAGMSG should not get automatic RPL_AWAY replies

Add WHO responses for services

Kiwi expects it due to https://github.com/kiwiirc/kiwiirc/pull/1244 , but
the corresponding spec change only altered the cap name, not the 005 name.

Unclear where we got draft/CHATHISTORY from, it looks like the merged drafts
have always used unprefixed CHATHISTORY as the token name.

* fix #2002

`CS AMODE #channel +f nickname` is invalid, but was being accepted
incorrectly.

* simplify logic

https://github.com/ircv3/ircv3-specifications/pull/508

According to the de facto standard, `AWAY :\r\n` is equivalent to `AWAY\r\n`.
Our behavior was inconsistent before, now it consistently matches the de facto
standard.

See #1994

WHO <nickname> should not respect +i

See 69448b13a1 / #1969; the compiler can now ensure that a uint64
intended for atomic access is always aligned to a 64-bit boundary.
Convert atomic operations on uint32s and pointers as well.

Sanitize ::1 to 0::1 in WHOX output

On a 32-bit architecture, 64-bit atomic loads and stores must be aligned to a
64-bit boundary. Since the (mysql.MySQL) struct is directly included in the
Server struct, it is impossible to guarantee this via the standard technique
of putting the 64-bit value at the beginning of the struct definition
(since the point at which it is included in the parent struct may cross a
64-bit boundary).

This optimization is probably pointless anyway, adding an additional
indirection won't make a difference.

NS SAREGISTER should send machine-readable responses. A simple approach:
check if the account-registration cap is enabled, and if so, send the
the same responses that would be sent by the REGISTER command.

See #1593; this enables a client-side implementation of bulk deletion

Reported by @Mikaela

* Add ACCEPT-tracking functionality (authorizing users to send DMs
  despite +R or other applicable restrictions)
* Sending a DM automatically accepts the recipient
* Add explicit ACCEPT command

Add more clarify in NS INFO and SAREGISTER about unregistered nicknames

Fixes #1924

KLINE'd clients would produce a QUIT snotice without a corresponding
CONNECT snotice; explicitly suppress the QUIT snotice.

Log it at level INFO for parity with connect / connect-ip