s4-dns: Ignore duplicate dns zones from multiple locations in dlz_bind9

s4-dns: Ignore duplicate dns zones from multiple locations in dlz_bind9

BIND DLZ module currently loads DNS zones from DNS partitions and domain
partitions using following prefixes:

   CN=MicrosoftDNS,DC=DomainDnsZones
   CN=MicrosoftDNS,DC=ForestDNSZones
   CN=MicrosoftDNS,CN=System

Windows supports DNS zones duplicated in DNS partitions and domain
partition and updates both of them simultaneously.

BIND DLZ module can handle DNS zones stored either in DNS partitions
or domain partition, but not both.  This patch ignores duplicate zones
from domain partition and allows BIND9 to work with AD with duplicate
DNS zones.

Signed-off-by: Amitay Isaacs <amitay@gmail.com>
Reviewed-by: Kai Blin <kai@samba.org>