This adds a NixOS module for XScreenSaver (from @aidalgol in #130218,
with a few updates).
The module:
* Installs XScreenSaver
* Sets up a suid wrapper for xscreensaver-auth
* Sets up a user service for xscreensaver
The suid wrapper should function correctly when xscreensaver is
installed via the derivation update in 40a00547b71.
Co-authored-by: Aidan Gauland <aidalgol@fastmail.net>
Co-authored-by: Anderson Torres <torres.anderson.85@protonmail.com>
