services: Improve the upower-configuration record.

Copy the defaults from the upower-service procedure to the
<upower-configuration> record type. This will allow making it the default
value for the upower-service-type, and deprecating the procedure. Export the
field accessors so that the <upower-configuration> record type becomes more
usable.

* gnu/services/desktop.scm (<upower-configuration>): Export it.
(upower-configuration-upower, upower-configuration-watts-up-pro?,
upower-configuration-poll-batteries?, upower-configuration-ignore-lid?,
upower-configuration-use-percentage-for-policy?,
upower-configuration-percentage-low, upower-configuration-percentage-critical,
upower-configuration-percentage-action, upower-configuration-time-low,
upower-configuration-time-critical, upower-configuration-time-action,
upower-configuration-critical-power-action): Add default and export.