openbsd_wlan: Basic adaptations to the net80211 layer.

The stack now compiles and links. Combining this with adaptations to
the driver gets things to the point where it runs scans, but we do
not yet implement ioctls so they can be returned to userland.

openbsd_wlan: Import OpenBSD net80211 layer and base dependencies.

Not yet wired into the build, of course.