* xmlschemas.c include/libxml/schemasInternals.h:
  Added an initial skeleton for indentity-constraints. This is all
  defined out, since not complete, plus it needs support from other
  modules.
  Added machanism to store element information for the
  ancestor-or-self axis; this is needed for identity-constraints
  and should be helpfull for a future streamable validation.
* include/libxml/xmlerror.h: Added an error code for
  identity-constraints.