parse: for option with posval, dump allowed values on failure

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Convert unit_base to posval[]

Signed-off-by: Jens Axboe <axboe@kernel.dk>
parse: don't print min/max option ranges if posval[] is set

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