provide a polkit D-Bus service that is activated on demand

provide a polkit D-Bus service that is activated on demand

Right now we provide two methods

 IsProcessAuthorized
 IsSystemBusNameAuthorized

This is useful for a couple of reasons

 - some mechanisms (e.g. Avahi) runs in a chroot and their only
   life-line to the world is a system bus connection. If it were to
   use libpolkit (and Lennart says he wants it to, yay!) it would need
   to bindmount crazy stuff into the chroot.

 - languages for which libpolkit bindings not yet exist can use
   this interface

Going forward, this service can expose a private interface meaning we
can get rid of (almost) all of our setgid helpers.