pre_read fixes

1) Add a specific runstate for pre-read, so we can see that this
   is what the job is currently doing.
2) open/close files for pre-read

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/HOWTO b/HOWTO
index 1390f38..e22f745 100644
--- a/HOWTO
+++ b/HOWTO
@@ -931,6 +931,7 @@
 P		Thread setup, but not started.
 C		Thread created.
 I		Thread initialized, waiting.
+	p	Thread running pre-reading file(s).
 	R	Running, doing sequential reads.
 	r	Running, doing random reads.
 	W	Running, doing sequential writes.