Add 'wait_for_previous' as an alias for the 'stonewall' option

Also update man page and HOWTO, plus add missing alias for
iomem_align while in there.

Signed-off-by: Jens Axboe <jaxboe@fusionio.com>
diff --git a/HOWTO b/HOWTO
index 5e9b390..88bd5e6 100644
--- a/HOWTO
+++ b/HOWTO
@@ -992,7 +992,8 @@
 		if verify_backlog_batch is larger than verify_backlog, some
 		blocks will be verified more than once.
 		
-stonewall	Wait for preceeding jobs in the job file to exit, before
+stonewall
+wait_for_previous Wait for preceeding jobs in the job file to exit, before
 		starting this one. Can be used to insert serialization
 		points in the job file. A stone wall also implies starting
 		a new reporting group.