We load registered channels unconditionally; reloading them again on rehash
is incorrect. This caused buggy behavior when channel registration was
disabled in the config, but some registered channels were already loaded.

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.

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

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.

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

* 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

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

RPL_WHOISCHANNELS didn't have proper line breaks

* optionally protect against multiple starts with flock

Fixes #1823

* use traditional .lock extension

* move config key to top level

Add a config option to suppress LUSERS

This reverts commit 5bbee02fe6.

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

RPL_WHOISACTUALLY should display some arbitrarily chosen IP address and
hostname.

Fixes #1733

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

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

Handle panics in rehash, even if it was triggered by SIGHUP

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

Track channel join times, use them to optionally enforce history access
restrictions

Fixes #810

This didn't work correctly for IPv6 or custom nets.
/UNDLINE IP can temporarily be used to reset the throttle.