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