* xmlschemas.c include/libxml/schemasInternals.h
  include/libxml/xmlerror.h: Initial implementation for
  redefinitions; this still misses checks for restrictions
  of the content model of complex types.
  Fixed default/fixed values for attributes (looks like they
  did not work in the last  releases).
  Completed constraints for attribute uses.
  Seperated attribute derivation from attribute constraints.
  Completed constraints for attribute group definitions.
  Disallowing <import>s of schemas in no target namespace if the
  importing schema is a chameleon schema. This contradicts
  the way Saxon, Xerces-J, XSV and IBM's SQC works, but the
  W3C XML Schema WG, thinks it is correct to dissalow such
  imports.
  Added cos-all-limited constraints.
  Restructured reference resolution to model groups and element
  declarations.
  Misc cleanup.