- Add three new functions to support circular run queues.

 - Add three new functions to support circular run queues.
 - runq_add_pri allows the caller to position the thread at any rqindex
   regardless of priority.
 - runq_choose_from() chooses the lowest priority thread starting from a given
   index.  The index is updated with the rqindex of the chosen thread.  This
   routine is used to pick the lowest priority relative to a given index.
 - runq_remove_idx() updates the index if the run queue that held the removed
   thread is now empty.