* Add a message truncation implementation
* Remove OPT if-statement at end of Scrub
* Impose RFC 6891 payload size limit in Scrub
* Remove *Msg receiver from truncateLoop
* Remove OPT record creation from Scrub
* Test that TestRequestScrubAnswerExact has correct record count
* Rename (*Msg).Scrub to Truncate
This better reflects it's purpose.
* Remove comment reference to scrubbing in Truncate
* Properly calculate the length of OPT record in Truncate
* Correct comment in IsEdns0 in regards to RFC 6891
* Handle the OPT record being anywhere in Truncate
* Slight cleanup of Msg.Truncate
Kian-Meng Ang
Tom Thorogood