sd-bus: add API to check if a client has privileges

This is a generalization of the vtable privilege check we already have,
but exported, and hence useful when preparing for a polkit change.

This will deal with the complexity that on dbus1 one cannot trust the
capability field we retrieve via the bus, since it is read via
/proc/$$/stat (and thus might be out-of-date) rather than directly from
the message (like on kdbus) or bus connection (as for uid creds on
dbus1).

Also, port over all code to this new API.