Bug#816872: wmbattery: memory leak in wmbattery

[David Johnson]

FYI: I've been running with this patch applied for over a week, no
issues seen.  Please apply unless there is an alternative.

David Johnson writes:
> 
> I've concluded libupower-glib is just super leaky.  The below patch
> moves the upower API calls into a child process that doesn't exist for
> long to keep the leaks out of the main process.
> 
> Tested against stretch versions, looks good.
>