the attached patch implements the changes described in #1468. The logic
itself is implemented in confdb_get_domain_internal, which breaks the
layering a little because there is some knowledge about the providers
used in the responders, in particular loading the flat name is only
called if the id_provider equals "AD".
Also technically the NetBIOS name could be completely different from the
AD domain name and could have been read from the rootDSE. But honestly
I really don't think it's worth it, so I went with a config option, that
would be unset in the vast majority of deployments.