Fix setting the MTU and speed via the command line

For FreeBSD use ioctl() to set the speed and mtu of the tun
interface instead of the Linux specific ethtool for the speed.
Also ensure that you can set the MTU to the default value in
case it is configured to a different one.
On non-FreeBSD platforms restore the old behaviour.
This fixes https://github.com/nplab/packetdrill/issues/17

gtests/net/packetdrill/netdev.c

@@ -111,8 +111,6 @@ static void check_remote_address(struct config *config,
 /* Create a tun device for the lifetime of this test. */
 static void create_device(struct config *config, struct local_netdev *netdev)
 {
-	char *command;
-
 	/* Open the tun device, which "clones" it for our purposes. */
 	int tun_fd = open(TUN_PATH, O_RDWR);
 	if (tun_fd < 0)
@@ -161,6 +159,23 @@ static void create_device(struct config *config, struct local_netdev *netdev)
 
 	DEBUGP("tun index: '%d'\n", netdev->index);
 
+#ifdef __FreeBSD__
+	struct tuninfo tuninfo;
+
+	if (ioctl(netdev->tun_fd, TUNGIFINFO, &tuninfo, sizeof(tuninfo)) < 0)
+		die_perror("TUNGIFINFO");
+	DEBUGP("Interface baudrate: %d bps, mtu: %d\n",
+	        tuninfo.baudrate, tuninfo.mtu);
+	DEBUGP("Requested baudrate: %ju bps, mtu: %d\n",
+	       IF_Mbps(config->speed), config->mtu);
+	if ((tuninfo.baudrate != IF_Mbps(config->speed)) ||
+	    (tuninfo.mtu != config->mtu)) {
+		tuninfo.baudrate = IF_Mbps(config->speed);
+		tuninfo.mtu = config->mtu;
+		if (ioctl(netdev->tun_fd, TUNSIFINFO, &tuninfo, sizeof(tuninfo)) < 0)
+			die_perror("TUNSIFINFO");
+	}
+#else
 	if (config->speed != TUN_DRIVER_SPEED_CUR) {
 		char *command;
 		asprintf(&command, "ethtool -s %s speed %u autoneg off",
@@ -179,10 +194,15 @@ static void create_device(struct config *config, struct local_netdev *netdev)
 		free(command);
 	}
 
-	asprintf(&command, "ifconfig %s mtu %d", netdev->name, config->mtu);
-	if (system(command) < 0)
-		die("Error executing %s\n", command);
-	free(command);
+	if (config->mtu != TUN_DRIVER_DEFAULT_MTU) {
+		char *command;
+		asprintf(&command, "ifconfig %s mtu %d",
+			 netdev->name, config->mtu);
+		if (system(command) < 0)
+			die("Error executing %s\n", command);
+		free(command);
+	}
+#endif
 
 	/* Open a socket we can use to configure the tun interface.
 	 * We only open up an AF_INET6 socket on-demand as needed,