Add list iterators over list elements

Address the comments from armink@rt-thread

include/rtservice.h

@@ -278,7 +278,7 @@ rt_inline int rt_slist_isempty(rt_slist_t *l)
  * @head:   the head for your single list.
  */
 #define rt_slist_for_each(pos, head) \
-    for (pos = (head)->next; &pos->next != (head); pos = pos->next)
+    for (pos = (head)->next; pos != RT_NULL; pos = pos->next)
 
 /**
  * rt_slist_for_each_entry  -   iterate over single list of given type