ioping: don't print 'unknown option' twice, show usage

getopt() already emits warning about unknown option, so if we do that, too,
the result will look like this:

	$ ./ioping -z
	./ioping: invalid option -- 'z'
	ioping: unknown option: -z

Don't warn about unknown option, instead show usage().
Signed-off-by: Kir Kolyshkin <kir@openvz.org>

ioping.c

@@ -248,7 +248,7 @@ void parse_options(int argc, char **argv)
 				path = optarg;
 				break;
 			case '?':
-				errx(1, "unknown option: -%c", optopt);
+				usage();
 		}
 	}