bsdinstall: fix prepopulating the ZFS disk menu with ZFSBOOT_DISKS

If the ZFSBOOT_DISKS variable is set to one or more disk names, then
those disks should be preselected in the disk menu.  However, the code
wasn't correctly setting the variable, leaving all disks unselected.

MFC after:	2 weeks
Sponsored by:	Axcient
Reviewed by:	dteske
Differential Revision: https://reviews.freebsd.org/D35331

usr.sbin/bsdinstall/scripts/zfsboot

@@ -659,7 +659,7 @@ dialog_menu_layout()
 		while :; do
 			# Loop over list of available disks, resetting state
 			for disk in $disks; do
-				f_isset _${disk}_status && _${disk}_status=
+				f_isset _${disk}_status && setvar _${disk}_status
 			done
 
 			# Loop over list of selected disks and create temporary
@@ -667,9 +667,8 @@ dialog_menu_layout()
 			for disk in $ZFSBOOT_DISKS; do
 				debug= f_device_find -1 \
 					$disk $DEVICE_TYPE_DISK disk
-				f_isset _${disk}_status ||
-					local _${disk}_status
-				_${disk}_status=on
+				local _${disk}_status
+				setvar _${disk}_status on
 			done
 
 			# Create the checklist menu of discovered disk devices