Import switchd(8), a basic WIP OpenFlow implementation for OpenBSD.

Import switchd(8), a basic WIP OpenFlow implementation for OpenBSD.

switchd consists of two parts:
1. switchd(8) and switchctl(8), an OpenFlow controller or "vswitch".
2. switch(4), an OpenFlow-aware kernel "bridge".

This the 1st part, the driver will be imported later.  The code will
remain disabled for a while, but it helps development to have it in
the tree.  switchd currently supports partial OpenFlow 1.0, but the
goal is to use OpenFlow 1.3.5 instead (switch(4) already does 1.3.5).

For more background information see:
http://www.openbsd.org/papers/bsdcan2016-switchd.pdf
https://youtu.be/Cuo0qT-lqig

With help from yasuoka@ goda@
Import discussed with deraadt@