Simultaneous exit/thread_destroy working

Reiterating again to simplify:

Working:
 - Pager issues destroy, client also issues exit
   they work in sync.

Missing
 - Pager killing itself
 - Pager killing all children while killing itself
 - Pager waiting on children

include/l4/generic/tcb.h

@@ -36,6 +36,7 @@ enum task_state {
 	TASK_INACTIVE	= 0,
 	TASK_SLEEPING	= 1,
 	TASK_RUNNABLE	= 2,
+	TASK_DEAD	= 3,
 };
 
 #define TASK_ID_INVALID			-1