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.

testModule.c

@@ -47,7 +47,7 @@ int main(int argc ATTRIBUTE_UNUSED, char **argv ATTRIBUTE_UNUSED) {
 
     /* build the module filename, and confirm the module exists */
     xmlStrPrintf(filename, sizeof(filename),
-                 (const xmlChar*) "%s/testdso%s",
+                 "%s/testdso%s",
                  (const xmlChar*)MODULE_PATH,
 		 (const xmlChar*)LIBXML_MODULE_EXTENSION);