pick_proc() called only just before returning to userspace

pick_proc() called only just before returning to userspace

- new proc_is_runnable() macro to test whether process is runnable. All tests
  whether p_rts_flags == 0 converted to use this macro

- pick_proc() calls removed from enqueue() and dequeue()

- removed the test for recursive calls from pick_proc() as it certainly cannot
  be called recursively now

- PREEMPTED flag to mark processes that were preempted by enqueueuing a higher
  priority process in enqueue()

- enqueue_head() to enqueue PREEMPTED processes again at the head of their
  current priority queue

- NO_QUANTUM flag to block and dequeue processes preempted by timer tick with
  exceeded quantum. They need to be enqueued again in schedcheck()

- next_ptr global variable removed