Ignore ns records from scanning

[theabraxas]

By default ns1, ns2, ns3...@domain.com records should not be scanned by nmap, or other tools apart from testing zone transfers (separate feature)

Phase one of this is to remove the ns# records from future scans (keep the record for reports).
Phase two of this is to enable an option, documented in help, which enables scanning these records if desired (should be very rare).