A reliable way for userspace to check if a msg is from kernel

- IPC_FLG_MSG_FROM_KERNEL status flag is returned to userspace if the
  receive was satisfied by s message which was sent by the kernel on
  behalf of a process. This perfectly reliale information.

- MF_SENDING_FROM_KERNEL flag added to processes to be able to set
  IPC_FLG_MSG_FROM_KERNEL when finishing receive if the receiver
  wasn't ready to receive immediately.

- PM is changed to use this information to confirm that the scheduling
  messages are indeed from the kernel and not faked by a process.

  PM uses sef_receive_status()

- get_work() is removed from PM to make the changes simpler

kernel/proc.h

@@ -221,6 +221,7 @@ struct proc {
 #define MF_USED_FPU	0x800   /* process used fpu during last execution run */
 #define MF_FPU_INITIALIZED	0x1000  /* process already used math, so fpu
 					 * regs are significant (initialized)*/
+#define MF_SENDING_FROM_KERNEL	0x2000 /* message of this process is from kernel */
 
 /* Scheduling priorities for p_priority. Values must start at zero (highest
  * priority) and increment.  Priorities of the processes in the boot image