allow dac_read_search and dac_override for dovecot/auth
This is needed for:
- /var/spool/postfix/private/ (postfix:root 700) -> dac_read_search
- /run/dovecot/auth-worker (dovecot:root 600) -> dac_override
References: https://bugzilla.opensuse.org/show_bug.cgi?id=1069470
I propose this patch for 2.9..trunk.