In the previous commit, the client would receive a failure message but would
actually remain logged in after an authzid/authcid mismatch. This was a
correctness rather than a security issue, but now it's fixed so that the client
never logs in in the first place.

* Fix #684
* Fix #683
* Add `CHANSERV CLEAR`
* Allow mode changes from channel founders even when they aren't joined
* Operators with the chanreg capability are exempt from max-channels-per-account
* Small fixes and cleanup

1. DRY "banner" descriptions of each services
2. `/HELP NICKSERV` now refers the user to `/NICKSERV HELP`, etc.
3. Fix double translation of `textGenerator` output

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).

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

The server should never crash during rehash, even if the config is invalid.