Add support for:

* a pfil AF_INET hook, so incoming ipv4 frames (post ethernet / vlan
  decap) can be inspected by the user code.
* Support transmitting raw frames out an interface.  It's a copy path;
  but it's better than not having any path.
Merge pull request #24 from erikarn/adrian_pfil_and_xmit

Because some people get all of libuinet and still just want raw packets.