Add 379 RPL_WHOISMODES

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.

0. Enable email verification
1. Register `dog`
2. Register `d0g`
3. Verify `dog`
4. Verify `d0g`: verification succeeds but the nick cannot be used

This affects RPL_WHOISACTUALLY and possibly others.

1. Avoid undefined behavior of time.Time{}.UnixNano()
2. Times should be compared with Equal()

ParseUserModeChanges and ParseChannelModeChanges already validate
that the modes are modifiable, so there's no need to repeat it here.

thanks @ajaspers!

Add chanmode preventing speech to error message.

Fixes #1182.