Fix some format string warnings with possible format string vulnerability

For https://bugzilla.gnome.org/show_bug.cgi?id=761029

Decorate every method in libxml2 with the appropriate
LIBXML_ATTR_FORMAT(fmt,args) macro and add some cleanups
following the reports.

include/libxml/parserInternals.h

@@ -351,7 +351,7 @@ XMLPUBFUN void XMLCALL
 						 xmlParserErrors xmlerr,
 						 const char *msg,
 						 const xmlChar * str1,
-						 const xmlChar * str2);
+						 const xmlChar * str2) LIBXML_ATTR_FORMAT(3,0);
 #endif
 
 /**