build switched to generate all RNG schemas from DTDs; now also

build switched to generate all RNG schemas from DTDs; now also
generates RNC versions from RNGs (you can use the RNCs for
context-sensitive, validated editing in Emacs/nXML mode, Oxygen
XML Editor, etc.); RNG and RNC conversions both rely on "trang";
tagged the directory (tag name: "original") before making the
change, so original CVS-maintained RNG files and union.xml and
Makefile that were in this directory can be checked out at any
time by doing a "cvs update -r original"