Declare 'prev' and 'this' outside the loop to avoid clang warning about 'prev' being uninitialized

Signed-off-by: Jens Axboe <axboe@kernel.dk>
Add FreeBSD cpu affinity support.

Since FreeBSD has different thread and process IDs, assume that fio_setaffinity
will always be called with a tid and fio_getaffinity with a pid.

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