Simplify for rDNS

[ainar-g]

It is currently hard to make a PTR query for rDNS. Firstly, the user has to manually reverse the domain name and add .in-addr.arpa. Secondly, it's particularly hard in the Windows command shell, where something like env often isn't readily available, so setting RRTYPE=PTR for just one query can become challenging.

There are two possible solutions I see:

1. Detect it when the first argument is an IP address and enter “rDNS mode”.
2. Add a command-line flag to enable said rDNS mode.