Implemented syscall_open

Added it to syscall handler
Removed syscall_exit

userprog/syscall.c

@@ -74,6 +74,9 @@ syscall_handler (struct intr_frame *f UNUSED)
   case SYSCALL_WRITE:
     syscall_write(f);
     break;
+  case SYSCALL_OPEN:
+    syscall_open(f);
+	break;
   default:
     printf ("WARNING: Invalid Syscall (%d)\n", syscall_number);
     thread_exit ();