Trying to get a final solution to the problem:

  - added back the old parser
  - make it the default
  - allow people to switch easilly
Few changes except the old parser where all names were changed and
made static except 3 entry points:
- Makefile.am parserold.c: added back the 1.8.11 parser for compatibility
- README: updated
- SAX.c: integrate back 1.8.11 callback code and call it when
  we are using the old parser
- entities.c: fixed a small problem
- parser.[ch]: allow to use either parser, add xmlUseNewParser(),
  and modify the Initialization code to check LIBXML_USE_NEW_PARSER
  or LIBXML_USE_OLD_PARSER
- tester.c: make it use the new parser by default, but check
  LIBXML_USE_OLD_PARSER and add the --oldparser option
Daniel