EDNS support

- Added type dnslib_opt_rr_t for representing the OPT RR.
- Removed type dnslib_edns_data_t (deprecated, duplicate)
- Changes to response to use the new type.
- Adding OPT RR to the response wire format if no additional
  RRSets were added.
- Changed reponse and edns API accordingly.
- Modified nameserver functions accordingly.
- Modified response tests to reflect the new API.
- Disabled EDNS tests - need significant modifications.

refs #458