iolog: check for thread termination in long delays

If we are doing a long delay now, fio won't be able to kill
the thread when you ctrl-c the job. Split the delay into .5s
intervals, and check for signal in between.

Signed-off-by: Jens Axboe <axboe@fb.com>
blktrace: fix missing line return in dprint()

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