add file_deny_write and file_allow_write to process creation and exiting to make executable read-only

src/threads/thread.h

@@ -127,6 +127,7 @@ struct thread
     struct process_result *result;      /* Result of the process. */
     struct list child_results;          /* List of children's of this thread
                                            process results. */
+    struct file *exec_file;             /* Thread's currently running file */
 
     /* Shared between thread.c and synch.c. */
     struct list_elem elem;              /* List element. */