s4:bind_dlz: Use the 'binddns dir' if possible

s4:bind_dlz: Use the 'binddns dir' if possible

The code makes sure we are backwards compatible. It will first check if
we still have files in the private directory, if yes it will use those.

If the the file is not in the private directory it will try the binddns
dir.

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Andrew Bartlet <abartlet@samba.org>