Implement priority donation helper function with propagation, w/ T

src/threads/thread.h

@@ -150,6 +150,8 @@ void thread_set_nice (int);
 int thread_get_recent_cpu (void);
 int thread_get_load_avg (void);
 
+void ready_list_reorder (void);
+
 /* Returns true iff the priority of the first list element's thread is greater
    than that of the second list element's thread. */
 list_less_func thread_priority_greater;