Disable status file updates on error

If fio doesn't have permission to unlink() the status file,
it'll continually dump status updates. If this happens, stop
doing status updates and log an error indicating why.

Signed-off-by: Jens Axboe <axboe@fb.com>
Bump length of description field to 256 chars

Ensure that string always end up \0 terminated, too.

Signed-off-by: Jens Axboe <axboe@fb.com>