Add OpenBSD support

I managed to make a clean compile and a few test runs using the
attached patches.

These work on OpenBSD 5.4 at least.  I used the 2.1.4 release of
fio as a basis.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
libfio: add missing OS's to fio_os_strings[]

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Update README to reflect that fio now runs on OpenBSD

Signed-off-by: Jens Axboe <axboe@kernel.dk>