Add file missed in last commit: Commit first pieces of port to OpenBSD,

    done by Martin Lexa (martin at martinlexa dot cz). Now that we've got
    porting for all three major BSDs (and the fourth being very similar to
    FreeBSD), move the mostly-duplication drm_os_* files into drmP.h.
    Remove some cruft from linux heritage and from pieces of the DRM that
    have since been removed.
Note that things are still not quite working for even FreeBSD, but these
    are first steps at cleanup, and just a WIP checkpoint.