service: Add support for debug controller
This exposes an object on D-Bus for controlling the debug output of the service, subject to authorization using polkit.
This adds a dependency on libpolkit-gobject, and bumps the GLib dependency to 2.72. If the right version of GLib is not available, the debug controller will be disabled.
Signed-off-by: Philip Withnall pwithnall@endlessos.org