Support DNSSEC resource record types

[jvns]

I noticed that the DNSSEC resource record types from RFC 4034 (DS, DNSKEY, RRSIG, NSEC) aren't supported.

I came across this when capturing DNS packets using dnspeep -- some of the queries the unbound resolver is making result in RRSIG and NSEC responses, which it's unable to parse.

Also thanks for this library, it's been great to work with.

[LinkTed]

DNSKEY and DS are already supported and NSEC is currently not done yet.