mass/async reverse DNS resolution is not case-insensitive

[richardschuetz]

Reverse DNS resolution in nmap fails if the PTR name in the DNS response's answer section is not all lower case (e.g. 52.0.2.10.IN-ADDR.ARPA instead of 52.0.2.10.in-addr.arpa). DNS labels should be treated case-insensitive.

[dmiller-nmap]

I'm sorry I did not remember this issue and PR. We had the issue re-reported in #2068, and I took action on that. I will add your name to the credit in the changelog since you reported this earlier.