Log Message:
- Adjust cdrom access logic so all the BSDs go to the BSD cdrom code,
not just FreeBSD and OpenBSD.

- Update NOT_FOR_PLATFORM: the only BSD for which the cdrom code can
actually work is DragonFly. The others are missing ioctls it uses.
This could probably be fixed up readily by someone who knows how cdrom
access is supposed to work, if any such person cares.

- Allow clang until proven otherwise.