This change introduces OPTResourse type to support DNS messages
containing various extension options as defined in RFC 6891.
Parsing and building OPT pseudo records requires allocations like TXT
records.

Also adds DNSSECAllowed, ExtendedRCode and SetEDNS0 methods to
ResourceHeader for convenience.

Updates golang/go#6464.
Updates golang/go#16218.

Change-Id: Ib72cea277201e4122c6b5effa320084ff351c886
Reviewed-on: https://go-review.googlesource.com/47170Reviewed-by: Ian Gudger <igudger@google.com>