[xml] relaxng support in libxml2

[Sebastian Rahtz <sebastian rahtz computing-services oxford ac uk>]

This is an excellent development, for which many thanks.
However, my schemas mostly fail, because I
use the back-compatibility ID and IDREF datatypes
(see http://www.oasis-open.org/committees/relax-ng/compatibility-20011203.html).

Would you consider implementing these too? The James Clark
and Sun implementations of Relax do so already.
-- 
Sebastian Rahtz      OUCS Information Manager
13 Banbury Road, Oxford OX2 6NN. Phone +44 1865 283431