debconf-show should not attempt to open passwords db if called with --db=config

Binary package hint: debconf

calling debconf-show as an unprivileged user with --db=config (which an unprivileged user has access to unlike the password db) always attemops to open the password database as well:

ogra@cmpc:~$ debconf-show --db=config tzdata
debconf: DbDriver "passwords" warning: could not open /var/cache/debconf/passwords.dat: Permission denied
  tzdata/Zones/Australia:
...

instead of attempting to open all DBs it should restrict itself to only use the DbDriver on the specificed database.