Use arg.h as much as possible and add usage() where missing

Rename eusage() to usage() as well.

printenv.c

@@ -1,14 +1,26 @@
 /* See LICENSE file for copyright and license details. */
 #include <stdio.h>
 #include <stdlib.h>
+#include "util.h"
 
 extern char **environ;
 
+static void
+usage(void)
+{
+	eprintf("usage: %s [variable...]\n", argv0);
+}
+
 int
 main(int argc, char **argv)
 {
 	char *var;
 
+	ARGBEGIN {
+	default:
+		usage();
+	} ARGEND;
+
 	if(argc == 1) {
 		while(*environ)
 			printf("%s\n", *environ++);