Import DNS64 support for unbound as implemented by Viagenie[1].

I have only maintained and forward ported the patch[2] since early
2011 and fixed one bug since.

Given NAT64/DNS64 seems to be one of the prefered transition technologies
I consider it important to support it.  As we are currently shipping
unbound in base and people have expressed interest in this patch I am
putting it out for testing.
I have not tested the in-tree version myself yet but run it out of ports
on older versions.
My hope is that upstream may pick it up and start shipping it in a new
version after all these years.

A noticable portion of the patch (in terms of lines) are only the
re-generated lexer and parser files these days.

Obtained from:	http://ecdysis.viagenie.ca/ http://www.viagenie.ca/ [1]
Obtained from:	http://ecdysis.viagenie.ca/download/ecdysis-unbound-1.4.7.20101117.patch.gz [2]
OKed by:	Marc Blanchet