Update thread to add exit_status, intialised to -1, into the thread structure, w/ E

src/threads/thread.c

@@ -659,6 +659,8 @@ init_thread (struct thread *t, const char *name, int nice, int priority,
   t->recent_cpu = recent_cpu;
   t->priority = t->base_priority;
 
+  t->exit_status = -1;
+
   old_level = intr_disable ();
   list_push_back (&all_list, &t->allelem);
   intr_set_level (old_level);

src/threads/thread.h

@@ -111,6 +111,7 @@ struct thread
     /* Shared between thread.c and synch.c. */
     struct list_elem elem;              /* List element. */
 
+    int exit_status;                    /* Exit Status: 0 = successful exit. */
 
 #ifdef USERPROG
     /* Owned by userprog/process.c. */