Add XDP Linux Functions

[nathanjsweet]

on behalf of @lmb

Add functions to attach an XDP program to a device (separate package?)

[nathanjsweet]

I'd love to merge it here, but if you want to make it a separate package that's fine too.

[lmb]

From a quick look attaching XDP requires sending netlink messages, which is the opposite of lightweight. We'd at least have to add a third party dependency, hence my suggestion to use a different package

[lmb]

Just found https://godoc.org/github.com/vishvananda/netlink#LinkSetXdpFdWithFlags which would work fine with our current interface. Seems like it doesn't define the flags that can be used though.

[lmb]

I think its worth doing this in a separate package, this would allow us to make e.g. supporting hardware offload nicer.