* 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.