Disk stat improvements

Add an option to disable the disk stats. And only dump the disk
stat header, if we actually have stats to show.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/HOWTO b/HOWTO
index 5fdc1f7..0b43871 100644
--- a/HOWTO
+++ b/HOWTO
@@ -648,6 +648,9 @@
 cpuchunks=int	If the job is a CPU cycle eater, split the load into
 		cycles of the given time. In milliseconds.
 
+disk_util=bool	Generate disk utilization statistics, if the platform
+		supports it. Defaults to on.
+
 
 6.0 Interpreting the output
 ---------------------------