Allow custom limit definitions that encompass multiple subnets

Convert the flat list of channels for always-on clients
to a map from channel names to channel-user modes.

Store the channel-user modes of always-on clients along with their
channel memberships, restore them on server startup. This will coexist
alongside /CS AMODE, which autoapplies modes to clients on join regardless
of their always-on status.

Allow `JOIN 0` with a confirmation code

Record INVITE in DM history for the benefit of offline always-on clients

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

Clients without event-playback should receive client-only tags in
replayed PRIVMSG / NOTICE.

Fixes #1407

Instead of building a new serialized message for each recipient,
try to cache them.

Resuming clients without the resume capability would receive
two JOIN lines per channel.

Support PROXY protocol v2, including ahead of plaintext connections

NS SUSPEND DEL incorrectly required the use of the casefolded account name.

Fixes #1380

Include an 'r' flag in RPL_WHOREPLY for registered nicks

Add 379 RPL_WHOISMODES

Enhancements to NS SUSPEND, including stored metadata and the ability
to list suspensions

DEOP is now pure syntactic sugar for /mode #channel -o nick,
the change is attributed to the originating user and not to ChanServ.

Allow messages-per-window to be 0

Address SETNAME UX problems reported by @KoraggKnightWolf in #oragono:

1. No feedback by default on success or failure
2. Multi-word realnames are not correctly interpreted by some clients

See #1171.