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/ip.go

@@ -0,0 +1,17 @@
+package main
+
+import (
+	"net"
+)
+
+const (
+	IPv4version   = 4
+	IPv4offsetSrc = 12
+	IPv4offsetDst = IPv4offsetSrc + net.IPv4len
+)
+
+const (
+	IPv6version   = 6
+	IPv6offsetSrc = 8
+	IPv6offsetDst = IPv6offsetSrc + net.IPv6len
+)