Implement process_wait.

src/threads/thread.h

@@ -36,9 +36,8 @@ struct process_result
 {
   tid_t tid;        /* The tid of the child process. */
   int exit_status;  /* The exit status of the child process. Initially set to
-                       -1, then to 0 when parent dies, or to exit_status when
-                       child dies (whichever happens first). */
-  struct lock lock; /* Lock to synchronise access to the exit_status. */
+                       -1, then to exit_status when child dies. */
+  struct lock lock; /* Lock to synchronise access to the status and sema. */
   struct semaphore sema; /* Semaphore to signal the parent that the exit_status
                             has been set. */
   struct list_elem elem; /* List element for the parent's children list. */