Add --section command line option

It enables the user to ship bigger and more complex job files, while
still allowing fio to only run a part of it.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>
diff --git a/fio.h b/fio.h
index 67f0bad..6dbc4cd 100644
--- a/fio.h
+++ b/fio.h
@@ -667,6 +667,7 @@
 extern unsigned long page_mask, page_size;
 extern int read_only;
 extern int eta_print;
+extern char *job_section;
 
 extern struct thread_data *threads;