Idle task never runs

- idle task becomes a pseudo task which is never scheduled. It is never put on
  any run queue and never enters userspace. An entry for this task still remains
  in the process table for time accounting

- Instead of panicing if there is not process to schedule, pick_proc() returns
  NULL which is a signal to put the cpu in an idle state and set everything in
  such a way that after receiving and interrupt it looks like idle task was
  preempted

- idle task is set non-preemptible to avoid handling in the timer interrupt code
  which make userspace scheduling simpler as idle task does not need to be
  handled as a special case.

kernel/main.c

@@ -178,6 +178,8 @@ PUBLIC void main()
 	
 	/* Set ready. The HARDWARE task is never ready. */
 	if (rp->p_nr == HARDWARE) RTS_SET(rp, RTS_PROC_STOP);
+	/* IDLE task is never put on a run queue as it is never ready to run */
+	if (rp->p_nr == IDLE) RTS_SET(rp, RTS_PROC_STOP);
 	RTS_UNSET(rp, RTS_SLOT_FREE); /* remove RTS_SLOT_FREE and schedule */
 	alloc_segments(rp);
   }