Revert "Do not use arg.h for tools which take no flags"

This reverts commit 9016d288f1.

Tools that have no options are required by POSIX to support "--" so
that conforming applications have a way to shield their operands from
implementations that provide options as an extension.

echo(1) is just an exception, so it is handled specially.

See OPTIONS in https://pubs.opengroup.org/onlinepubs/9699919799.2018edition/utilities/V3_chap01.html#tag_17_04

cksum.c

@@ -92,12 +92,21 @@ cksum(int fd, const char *s)
 	putchar('\n');
 }
 
+static void
+usage(void)
+{
+	eprintf("usage: %s [file ...]\n", argv0);
+}
+
 int
 main(int argc, char *argv[])
 {
 	int fd;
 
-	argv0 = *argv, argv0 ? (argc--, argv++) : (void *)0;
+	ARGBEGIN {
+	default:
+		usage();
+	} ARGEND
 
 	if (!argc) {
 		cksum(0, NULL);