Change exit (0)' to exit (EXIT_SUCCESS)',

`exit (1)' to `exit (EXIT_FAILURE)', and
`usage (1)' to `usage (EXIT_FAILURE)'.

src/expr.c

@@ -175,7 +175,7 @@ main (int argc, char **argv)
   if (argc == 1)
     {
       error (0, 0, _("too few arguments"));
-      usage (1);
+      usage (EXIT_FAILURE);
     }
 
   args = argv + 1;