Preserved variables not so obviously ignoring changes in PAM should be allowed to be overridden through another option

Changes in the environment made by PAM may also need to be respected sometimes so I suggest adding another option to allow that.

It's also practical to forward existing values to PAM to allow its modules to respect existing values when applying default values.  See my similar issue on su: https://github.com/util-linux/util-linux/issues/3329.