Add FreeBSD support

Signed-off-by: Brady OBrien <brady.obrien128@gmail.com>

tun_freebsd_386.go

@@ -0,0 +1,18 @@
+/* SPDX-License-Identifier: GPL-2.0
+ *
+ * Copyright (C) 2017-2018 Jason A. Donenfeld <Jason@zx2c4.com>. All Rights Reserved.
+ */
+
+/* Types to deal with FreeBSD fdiogname ioctl for determining tun device name */
+
+package main
+
+// Iface name max len
+const _IFNAMESIZ = 16
+
+// structure for iface requests with a pointer
+type ifreq_ptr struct {
+	Name [_IFNAMESIZ]byte
+	Data uintptr
+	Pad0 [20]byte
+}