Fixed MTU method for linux TUN interface

Updated the TUN interface
Added the "MTU" method for the linux implementation of the TUN interface

src/tun.go

@@ -1,8 +1,8 @@
 package main
 
 type TUNDevice interface {
-	Read([]byte) (int, error)
-	Write([]byte) (int, error)
-	Name() string
-	MTU() int
+	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)
+	MTU() (int, error)         // returns the MTU of the device
+	Name() string              // returns the current name
 }