Enabled IDC parsing and validation. Added xmlSchemaCopyValue to the API;
* xmlschemas.c: Enabled IDC parsing and validation. * xmlschemastypes.c include/libxml/xmlschemastypes.h: Added xmlSchemaCopyValue to the API; this was done due to validation of default attributes against IDCs: since IDC keys consume the precomputed value, one needs a copy. * pattern.c: Enabled IDC support; this is currently done via calling xmlPatterncompile with a flag arg of 1.