Add support for modifying the randomness of a workload

Fio supports random or sequential IO, or random IO with a number of
sequential IOs in between. The percentage_random options allows
more fine grained control over this. It defaults to 100, which would
make any random workload still purely random. If set below 100,
there will be some percentage of sequential IOs. This happens randomly,
but at the specified percentages.

The percentage_sequential option is just the complement of that, it
is guaranteed that:

	percentage_sequential + percentage_random == 100%

The later setting has priority, each will adjust the other.

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