Add XFRM support

[zooknotic]

I've added some code to support the netlink xfrm API, primarily to support management of IPsec tunnels. Similar to the other protocols, I've split it up into netlink-packet-xfrm for message parse/emit, and xfrmnetlink for request/response handling. Feel free to add to your project if desired, or let me know if there's anything that should be changed to make it more suitable for inclusion. Thx.