Auth worker fix

It's handy to be able to set the auth-worker user. It's even handier to be able to set the auth-worker group. If  the shadow file needs to be read by the auth-worker (for system accounts), the group can be set to 'shadow' (you could also set the user to root, but the former is preferable). This is per the [Dovecot Wiki](http://wiki2.dovecot.org/UserIds).

The title of the auth-worker user resource was adjusted, as `master` didn't make sense. Also, the require on `userdblistener1` seems wholly unnecessary to me; I was able to re-apply without problem, but it may be wise to test this on a fresh install. Still, auth-worker user and group aren't nested under anything in `conf.d/10-master.conf`, which is the only thing I know of that would necessitate a require dependency.

P.S. This PR is clean too.