dirname: support --

POSIX-2017 clarifies that -- and normal option parsing must be supported.
See EXAMPLES in basename(1p)

Signed-off-by: Mattias Andrée <maandree@kth.se>

dirname.c

@@ -13,7 +13,10 @@ usage(void)
 int
 main(int argc, char *argv[])
 {
-	argv0 = *argv, argv0 ? (argc--, argv++) : (void *)0;
+	ARGBEGIN {
+	default:
+		usage();
+	} ARGEND
 
 	if (argc != 1)
 		usage();