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/proc.c

@@ -155,7 +155,19 @@ not_runnable_pick_new:
 	/* this enqueues the process again */
 	if (proc_no_quantum(proc_ptr))
 		RTS_UNSET(proc_ptr, RTS_NO_QUANTUM);
-	proc_ptr = pick_proc();
+
+	/*
+	 * if we have no process to run, set IDLE as the current process for
+	 * time accounting and put the cpu in and idle state. After the next
+	 * timer interrupt the execution resumes here and we can pick another
+	 * process. If there is still nothing runnable we "schedule" IDLE again
+	 */
+	while (!(proc_ptr = pick_proc())) {
+		proc_ptr = proc_addr(IDLE);
+		if (priv(proc_ptr)->s_flags & BILLABLE)
+			bill_ptr = proc_ptr;
+		halt_cpu();
+	}
 
 check_misc_flags:
 
@@ -1388,7 +1400,6 @@ PRIVATE struct proc * pick_proc(void)
 		bill_ptr = rp;		/* bill for system time */
 	return rp;
   }
-  minix_panic("no runnable processes", NO_NUM);
   return NULL;
 }