adding the tree debug mode fixing various problems reported by the debug

* debugXML.c include/libxml/debugXML.h include/libxml/xmlerror.h:
  adding the tree debug mode
* parser.c relaxng.c tree.c xpath.c: fixing various problems reported
  by the debug mode.
* SAX2.c: another tree fix from Rob Richards
Daniel
diff --git a/xpath.c b/xpath.c
index c55958d..89a8e28 100644
--- a/xpath.c
+++ b/xpath.c
@@ -254,7 +254,7 @@
 }
 
 /**
- * xmlXPathErrMemory:
+ * xmlXPathPErrMemory:
  * @ctxt:  an XPath parser context
  * @extra:  extra informations
  *