Check for filename clashes /w dnssec-policy zones

Check for filename clashes /w dnssec-policy zones

Just like with dynamic and/or inline-signing zones, check if no two
or more zone configurations set the same filename. In these cases,
the zone files are not read-only and named-checkconf should catch
a configuration where multiple zone statements write to the same file.

Add some bad configuration tests where KASP zones reference the same
zone file.

Update the good-kasp test to allow for two zones configure the same
file name, dnssec-policy none.