dns scavenging: avoid passing blobs

dns scavenging: avoid passing blobs

We can construct these ldb values for each zone with minimal cost and
it is less bamboozling to do so.

While we're fiddling with signatures, we can make this used-once local
function static.

Signed-off-by: Douglas Bagnall <douglas.bagnall@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>