xauth: add a configuration string option to be passed to XAuth instances

The configuration string is appended to the XAuth backend name, separated by
a colon. The configuration string is passed untouched to the backend, where
it can change the behavior of the XAuth module.

src/libcharon/sa/ikev1/tasks/xauth.c

@@ -127,7 +127,7 @@ static xauth_method_t *load_method(private_xauth_t* this)
 	{
 		if (name)
 		{
-			DBG1(DBG_CFG, "no XAuth method found named '%s'", name);
+			DBG1(DBG_CFG, "no XAuth method found for '%s'", name);
 		}
 		else
 		{