compel: error out on unknown opt or missing arg

getopt_long() prints an error message and returns '?' in cases
 - an unknown option is given
 - a required option argument is missing

In such cases, we need to show usage and exit with an error.
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>

compel/src/main.c

@@ -201,7 +201,9 @@ int main(int argc, char *argv[])
 			       COMPEL_SO_VERSION_SUBLEVEL);
 			exit(0);
 			break;
-		default:
+		default: // '?'
+			// error message already printed by getopt_long()
+			return usage(1);
 			break;
 		}
 	}