OpenBSD-to-FreeBSD/Haiku compatibility headers.

OpenBSD's and FreeBSD's kernel APIs are very similar, but there are
still some differences. Fortunately these are small enough that we
can take care of most of them with a header-only shim layer.

(The files and functions implemented by this commit were implemented
and tested against the modules and drivers imported and adapted in
the following commits. Some of this took quite a bit of time to get
right, especially where the APIs have diverged in very subtle ways.)