Create the renamed CV as a shallow copy of the original one

From now they share the exact same pad, without any refcount bump. This
means that we have to handle the renamed CV destruction ourselves by
clearing its padlist entry before freeing it, but we need to reset its
depth anyway before that (or that would cause "still in use" warnings).

This allows us to call pp_entersub directly with the renamed CV as we were
doing earlier, and to remove the exotic destruction dance from
su_uplevel_restore().