backlight: add minimal tool to save/restore screen brightness across reboots

As many laptops don't save/restore screen brightness across reboots,
let's do this in systemd with a minimal tool, that restores the
brightness as early as possible, and saves it as late as possible. This
will cover consoles and graphical logins, but graphical desktops should
do their own per-user stuff probably.

This only touches firmware brightness controls for now.