power: Fix "When battery power is critical" label with UPower 1.0

With UPower 1.0, the critical action to take when the battery level is
too low isn't in the hands of the user anymore, but in the hands of the
system.

It's now handled by UPower directly, through the
CriticalPowerAction configuration option. Instead of asking logind
whether the system supports hibernation, ask UPower directly what the
action will be when we're nearly out of battery.

https://bugzilla.gnome.org/show_bug.cgi?id=749436