Replace all CSS2 copies with a single css2/ equal to the 2011/6 REC

This is from the css21/ directory in
b739b88 with changes to variables in
the Makefile to make the build exactly match
https://www.w3.org/TR/2011/REC-CSS2-20110607 ("W3C Recommendation
07 June 2011, edited in place 12 April 2016 to point to new work")

Author: gsnedders