gfio: add support for FIO_OPT_INT options with posval

Signed-off-by: Jens Axboe <axboe@kernel.dk>
options: make unit_base be posval[] based

We can kill the verifier and ensure we only get the
possible values we need.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
parse: add posval support to FIO_OPT_INT

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