Add RFC 6891 EDNS(0) Support

Implement encoding and decoding for the EDNS(0) OPT pseudo-RR as defined in RFC 6891. - Allows magna to parse messages signaling extended capabilities (e.g., larger UDP buffer sizes, DNSSEC OK (DO) bit). - Allows magna to generate messages including an OPT RR. - Includes parsing and setting the DNSSEC-related AD and CD header bits (RFC 4035), often used with EDNS(0).