* xmlschemas.c: Added "Particle correct 2" to parsing of model groups.
  Added handling substitution groups inside <choice> and <sequence>;
  for <all> this is not supported yet. Changed circular checks for
  model groups definitions. "memberTypes" are processed at different
  levels now: component resolution first, construction later; this
  goes hand in hand with a global change to handle component
  resolution in a distinct phase. Fixed invalid default values for
  elements to mark the schema as invalid; this just resulted in an
  error report previously, but the schema was handled as valid.
  Separated the assignment of the model groups to referencing
  model group definition references (i.e. particles); this was
  needed to perform the circularity check for model group definitions.
  Added "Element Declaration Properties Correct (e-props-correct)"
  constraints. Separated component resolution for simple/complex
  types.
* include/libxml/schemasInternals.h: Added a flag for substitution
  group heads.