comments cleanups use xmllint for doing the RelaxNG tests preparing 2.5.2

* HTMLparser.c tree.c xmlIO.c: comments cleanups
* Makefile.am: use xmllint for doing the RelaxNG tests
* configure.in: preparing 2.5.2 made schemas support default to
  on instead of off
* relaxng.c: removed the verbosity
* xmllint.c: added --relaxng option
* python/generator.py python/libxml_wrap.h: prepared the integration
  of the new RelaxNG module and schemas
* result/relaxng/*: less verbose output
Daniel

xmlIO.c

@@ -391,6 +391,8 @@ xmlFileOpen_real (const char *filename) {
  *
  * Wrapper around xmlFileOpen_real that try it with an unescaped
  * version of @filename, if this fails fallback to @filename
+ *
+ * Returns a handler or NULL in case or failure
  */
 void *
 xmlFileOpen (const char *filename) {