desktop: split lxde and xfce from xsession
desktop: disable xscreensaver via site config

This sets the xscreensaver mode to blank in the site config file. It's a
little better than puppeting the entire user .xscreensaver file since we
can hopefully maintain better compatibility across versions.

(Ideally we'd use augeas instead of sed, but the config file format
looks crazy so I don't even want to attempt it)