Enable running on D-Bus system daemon

[Matt Hoosier]

> Since GeoClue uses D-BUS as the communication protocol between
> clients, master and backends, it IMO should be possible to write the
> clients and backends in any language that supports D-BUS instead of
> having to write language bindings to the C API.

I think that's fine.

> So far this has been possible, and so I'd prefer solving the bus
> question in a way that apps written in different languages can use.
>
> How would that GConf idea sound to you? Is that feasible in the
> platform you're working on?

I don't see a problem. How does the following sound? We retain the
"--enable-system-bus" switch, but it just alters the default value of
a GConf key:

  /apps/geoclue/config/systemwide: boolean

which is then consulted internally and externally to determine which
bus should be used.