nixos/upower: Allow customization of UPower.conf

Documentation of options and default values taken from the upstream
UPower.conf. The documentation was modified slightly to make more sense
when displayed on https://nixos.org/nixos/options.html.

A copy of upstream UPower.conf can be found here:
https://cgit.freedesktop.org/upower/tree/etc/UPower.conf?id=28bd86c181e2510ef6a1dc7cfa26f97803698a79