fix a problem reported by Ashwin for system parameter entities referenced

* parser.c: fix a problem reported by Ashwin for system parameter
  entities referenced from entities in external subset, add a
  specific loading routine.
* test/valid/dtds/external.ent test/valid/dtds/external2.ent
  test/valid/t11.xml result/valid/t11.xml*: added the test to
  the regression suite
Daniel

svn path=/trunk/; revision=3713
7 files changed