Don't crash when printing name of nil interface
Improve debug messages
Ensure we only attempt to handle TypeA queries
Extend tests slightly
Look up by IP
Add simple rDNS lookup
Removed duplicate check for rDNS domain

There's no need to check whether the queried address is a subdomain of
in-addr.arpa, since this handler is registered with the muxer under
that domain anyway.

(Also, remove a magic number)