DNS: Warn if forwarding policy conflicts with automatic empty zones
    
    Forwarding policy "first" or "none" may conflicts with some automatic empty
    zones. Queries for zones specified by RFC 6303 will ignore
    forwarding and recursion and always result in NXDOMAIN answers.
    
    This is not detected and warned about. Global forwarding is equivalent
    to forward zone ".".
    
    Example:
    Forward zone 1.10.in-addr.arpa with policy "first"
    will not forward anything because BIND will automatically prefer
    automatic empty zone "10.in-addr.arpa." which is authoritative.
    
    https://fedorahosted.org/freeipa/ticket/5710
    
    Reviewed-By: Martin Basti <mbasti@redhat.com>