Make Interface.Inside an interface type (#252)

This commit updates the Interface.Inside type to be a new interface
type instead of a *Tun. This will allow for an inside interface
that does not use a tun device, such as a single-binary client that
can run without elevated privileges.

tun_linux.go

@@ -288,6 +288,14 @@ func (c Tun) Activate() error {
 	return nil
 }
 
+func (c *Tun) CidrNet() *net.IPNet {
+	return c.Cidr
+}
+
+func (c *Tun) DeviceName() string {
+	return c.Device
+}
+
 func (c Tun) advMSS(r route) int {
 	mtu := r.mtu
 	if r.mtu == 0 {