Setting auth-worker user

Yeah, again, *really* sorry these aren't on clean branches by themselves; I'm usually much better about that. If need be, I can pick changes and create clean branches for you.

Anyway, this lets us set the auth-worker user parameter, which needs to be 'root' if we're using PAM for system accounts (a thing usually done in single-user setups).

I'm learning way too much about Dovecot and PAM lately.