Some more tidying of authtracker and related things.

Authtracker used to have 2 ways of getting userID - either from np->auth or via the chanserv macro.  These now actually do the same thing so aren't both needed any more.
Also removed the HOOK_CHANSERV_SETUSERID hook as this is now redundant - the userid is always set at ACCOUNT time (for local or remote auths) - we ignore remote ACs without userID set now.
Also a slight tidy of the condition for autocreating auths - we just look for the chanserv extension attached to the user rather than looking up the account name.