Api: Fix VPN filtering on KitKat

Due to firewall rule changes in KitKat's netd, the outbound interface for
VPN-bound packets shows up as e.g. wlan0 or rmnet0, NOT as tun0.  Add a
new rule to make sure they get handled properly.

See also:
http://forum.xda-developers.com/showthread.php?p=48703545