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/fio.h b/fio.h
index cb80a11..b1360dd 100644
--- a/fio.h
+++ b/fio.h
@@ -919,6 +919,7 @@
 	TD_INITIALIZED,
 	TD_RAMP,
 	TD_RUNNING,
+	TD_PRE_READING,
 	TD_VERIFYING,
 	TD_FSYNCING,
 	TD_EXITED,