Simplify tty(1)

Add a usage line and print "not a tty" for all error cases.

tty.c

@@ -1,24 +1,26 @@
 /* See LICENSE file for copyright and license details. */
-#include <errno.h>
+#include <unistd.h>
 #include <stdio.h>
 #include <stdlib.h>
-#include <unistd.h>
+#include "util.h"
+
+static void
+usage(void)
+{
+	eprintf("usage: %s\n", argv0);
+}
 
 int
-main(void)
+main(int argc, char *argv[])
 {
 	char *tty;
 
-	if((tty = ttyname(STDIN_FILENO))) {
+	ARGBEGIN {
-		puts(tty);
+	default:
-		return 0;
+		usage();
-	}
+	} ARGEND;
-	else if(errno == ENOTTY) {
+
-		puts("not a tty");
+	tty = ttyname(STDIN_FILENO);
-		return 1;
+	puts(tty ? tty : "not a tty");
-	}
+	return tty ? 0 : 1;
-	else {
-		perror("ttyname");
-		return 2;
-	}
 }