find: Exit if there is an unknown option.

Ignoring the parameter with the unknown options is unlikely to be what was
intended.

Example:
  find -n .

Note that things like
  find -n
already caused an exit, equivalent to "find" by itself.

usr.bin/find/main.c

@@ -120,7 +120,7 @@ main(int argc, char *argv[])
 			break;
 		case '?':
 		default:
-			break;
+			usage();
 		}
 
 	argc -= optind;