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.

* Defer account login snomask until after connection registration is complete
* Don't send account-notify for pre-registration clients

Fixes #1407

Support PROXY protocol v2, including ahead of plaintext connections

Add 379 RPL_WHOISMODES

See discussion on #68.

Previously, we were applying defaults before the user had completed
registration. This meant that the number of invisible users was
incremented when the user connected, and then the total was incremented
when registration was completed.

Now both counters are updated at the same time. If a user disconnects
prior to registration, +i has not yet been applied so it would not be
decremented.