First set of code review patches

src/tun.go

@@ -9,6 +9,7 @@ const DefaultMTU = 1420
 type TUNDevice interface {
 	Read([]byte) (int, error)  // read a packet from the device (without any additional headers)
 	Write([]byte) (int, error) // writes a packet to the device (without any additional headers)
+	IsUp() (bool, error)       // is the interface up?
 	MTU() (int, error)         // returns the MTU of the device
 	Name() string              // returns the current name
 }