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/options.c b/options.c
index ffe5470..82e3e27 100644
--- a/options.c
+++ b/options.c
@@ -1879,6 +1879,7 @@
 	},
 	{
 		.name	= "stonewall",
+		.alias	= "wait_for_previous",
 		.type	= FIO_OPT_STR_SET,
 		.off1	= td_var_offset(stonewall),
 		.help	= "Insert a hard barrier between this job and previous",