Stop auto-creating PTR records when CNAME exists in reverse zone
Stop auto-creating PTR records when CNAME exists in reverse zone

Classless IN-ADDR.ARPA delegation (RFC-2317) delegates PTR management to
a separate zone. Where such delegation has been set up we should not
attempt to instruct PowerDNS to create a corresponding PTR record.

Resolves: #45

operasoftware/dns-ui,operasoftware/dns-ui