Provides access to the Linux AF_XDP socket family.  For more
information on AF_XDP, see:
https://www.kernel.org/doc/html/latest/networking/af_xdp.html.

Change-Id: I7efb82c4a67da3e15932e3c0d882f0bad7ba16ef
Reviewed-on: https://go-review.googlesource.com/136695
Run-TryBot: Matt Layher <mdlayher@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>