RFE: optionally, send along selected creds on all method calls

[poettering]

On the system bus most method calls are nowadays authorized via polkit. Which is slow, due to the many roundtrips involved. Would love if we could cut this down a bit. hence, would love it if we could ask broker to attach implicitly selected creds info (i.e. uid, …) to method calls as they propagate through the broker, so that we don't have to ask explicitly for that all the time.