Exit with usage when extra arguments are on command line

preventing mistakes such as "halt 0p" for "halt -p".
Approved by:	bde (mentor), phk (mentor)
MFC after:	1 week

sbin/reboot/reboot.c

@@ -116,6 +116,8 @@ main(int argc, char *argv[])
 		}
 	argc -= optind;
 	argv += optind;
+	if (argc != 0)
+		usage();
 
 	if ((howto & (RB_DUMP | RB_HALT)) == (RB_DUMP | RB_HALT))
 		errx(1, "cannot dump (-d) when halting; must reboot instead");