bsdinstall: Fix local_unbound option default on revisit

The variable used for the checklist's default value needs to correspond
to the rc.conf variable as that's what's being parsed to determine them.
In the case of local_unbound it's missing the _enable suffix and thus
always defaults to off on revisit.

Fixes: 58eb9abb3157 ("Add a line to the post-installation configuration dialog to enable the local_unbound service.")