Although a peer address MUST be specified for IPv4, it MUST NOT
be specified when using IPv6, when configuring the IP address
on an utun device using ifconfig.

This requires the tunneling devices to be used in P2P mode. So
pass the gateway address around.
This is not working:
* Can't configure IPv6 addresses on utun devices.
* Can't use libpcap on utun devices
* Can't use libpcap as currently being used (taking the direction
  into account.

Initial commit for the packetdrill network stack testing tool:

  https://code.google.com/p/packetdrill/



Signed-off-by: Neal Cardwell <ncardwell@google.com>