Iterate through all records with DNSSEC NSEC records

[HarryR]

As part of the discovery phase we need to check if the domain returns NSEC records instead of NSEC3 records.

Then use these to iterate through all records in the domain.

See: http://strotmann.de/roller/dnsworkshop/entry/take_your_dnssec_with_a which references the ldns-walk utility.