bsdinstall: hook up help line and prompt for ZFS disk selection

Previously we were passing the wrong variable names for the prompt and
help line, so the intended action wasn't clear to the user.

Reviewed by:	jhb, markj
MFC after:	3 days
See also:	opnsense/installer#22
Pull Request:	freebsd#1579

usr.sbin/bsdinstall/scripts/zfsboot

@@ -688,17 +688,17 @@ dialog_menu_layout()
 			done
 
 			eval f_dialog_checklist_size height width rows \
-				\"\$title\" \"\$btitle\" \"\$prompt\" \
-				\"\$hline\" $disk_check_list
+				\"\$title\" \"\$btitle\" \"\$disk_prompt\" \
+				\"\$disk_hline\" $disk_check_list
 
 			selections=$( eval $DIALOG \
 				--title \"\$DIALOG_TITLE\"         \
 				--backtitle \"\$DIALOG_BACKTITLE\" \
 				--separate-output                  \
-				--hline \"\$hline\"                \
+				--hline \"\$disk_hline\"           \
 				--ok-label \"\$msg_ok\"            \
 				--cancel-label \"\$msg_back\"      \
-				--checklist \"\$prompt\"           \
+				--checklist \"\$disk_prompt\"      \
 				$height $width $rows               \
 				$disk_check_list                   \
 				2>&1 >&$DIALOG_TERMINAL_PASSTHRU_FD