Beginning work on TUN interface

And outbound routing

I am not entirely convinced the use of net.IP is a good idea,
since the internal representation of net.IP is a byte slice
and all constructor functions in "net" return 16 byte slices
(padded for IPv4), while the use in this project uses 4 byte slices.
Which may be confusing.

src/tun.go

@@ -0,0 +1,8 @@
+package main
+
+type TUN interface {
+	Read([]byte) (int, error)
+	Write([]byte) (int, error)
+	Name() string
+	MTU() uint
+}