Allow IRCds to log users out

[ValwareIRC]

Please can Atheme support incoming logout requests.

in UnrealIRCd's case just supporting incoming SVSLOGIN * nick 0.

This allows logouts to happen on the IRCd through a /LOGOUT server-side alias or something, which might be useful during netsplits or in cases where services are down, a user may still log out, so supporting allowing the IRCd to log the user out can be helpful.

When doing a SASL, the client knows nothing about services, it makes sense to support a logout command where services doesn't even need to be online for the logout to happen.