Independently of this, ClientLookupSet.Resume ensures that at most one
resume can succeed, so this doesn't actually change the behavior.
But ResumeManager should be a standalone example of how to implement
resume without race conditions.

See discussion on #391.

reported by bogdomania

1. KICK without privileges incorrectly returned ERR_CANNOTSENDTOCHAN
2. Halfops should be able to kick voice and unprivileged, but not other halfops

Standard nickserv clients like znc's expect (by default) to send just:

PRIVMSG NickServ :identify $passphrase

with the account name assumed to be the currently held nick.

Let's support this in the common case where the client doesn't have a certfp
(if it does, the first argument is the account name, not the passphrase).

reported by bogdomania

Closes #355

Fixes #353.

* limit the total number of registrations per account
* when an account is unregistered, unregister all its channels