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/table.c

@@ -45,7 +45,14 @@
 PUBLIC char *t_stack[TOT_STACK_SPACE / sizeof(char *)];
 	
 /* Define flags for the various process types. */
-#define IDL_F 	(SYS_PROC | PREEMPTIBLE | BILLABLE)	/* idle task */
+#define IDL_F 	(SYS_PROC | BILLABLE)	/* idle task is not preemptible as we
+					 * don't want it to interfere with the
+					 * timer tick interrupt handler code.
+					 * Unlike other processes idle task is
+					 * handled in a special way and is
+					 * preempted always if timer tick occurs
+					 * and there is another runnable process
+					 */
 #define TSK_F 	(SYS_PROC)				/* kernel tasks */
 #define SRV_F 	(SYS_PROC | PREEMPTIBLE)		/* system services */
 #define VM_F 	(SYS_PROC)				/* vm  */
@@ -113,7 +120,7 @@ PRIVATE int
 
 PUBLIC struct boot_image image[] = {
 /* process nr, pc,flags, qs,  queue, stack, traps, ipcto, call,  name */ 
-{IDLE,  idle_task,IDL_F,  8, IDLE_Q, IDL_S,     0,     0, no_c,"idle"  },
+{IDLE,       NULL,IDL_F,  0, IDLE_Q, IDL_S,     0,     0, no_c,"idle"  },
 {CLOCK,clock_task,TSK_F,  8, TASK_Q, TSK_S, TSK_T,     0, no_c,"clock" },
 {SYSTEM, sys_task,TSK_F,  8, TASK_Q, TSK_S, TSK_T,     0, no_c,"system"},
 {HARDWARE,      0,TSK_F,  8, TASK_Q, HRD_S,     0,     0, no_c,"kernel"},