Refactor OPT handling code into OPT record class. (#520)

Refactor OPT handling code into OPT record class. (#520)

* Create an OPT record class.

* Move OPT logic to one place.

* Store the OPT record on the message object.

This also adds a Renderer.add_rdata() method.

* Add Rdataset.rdata_to_wire() helper.

* Fix conflicts; simplify.

* Fix typo.

* style

* Add a trivial to_text so that repr() works.

* Add _parse_special_rr_header

* More OPT checking.

Pass the name to _parse_rr_header and _parse_special_rr_header, and
check that the OPT record has the root name.