Removed IFF_NO_PI from TUN linux

This change was needed for the Linux TUN status hack
to work properly (not increment the error counter).

This commit also updates the TUN interface to allow for
the construction / removal of the TUN info headers in-place.

src/send.go

@@ -127,8 +127,9 @@ func (device *Device) RoutineReadFromTUN() {
 
 		// read packet
 
-		elem.packet = elem.buffer[MessageTransportHeaderSize:]
-		size, err := device.tun.device.Read(elem.packet)
+		offset := MessageTransportHeaderSize
+		size, err := device.tun.device.Read(elem.buffer[:], offset)
+
 		if err != nil {
 			logError.Println("Failed to read packet from TUN device:", err)
 			device.Close()
@@ -139,7 +140,7 @@ func (device *Device) RoutineReadFromTUN() {
 			continue
 		}
 
-		elem.packet = elem.packet[:size]
+		elem.packet = elem.buffer[offset : offset+size]
 
 		// lookup peer