bus-util: simplify bus_verify_polkit_async() a bit

First, let's drop the "bus" argument, we can determine it from the
message anyway.

Secondly, determine the right callback/userdata pair automatically from
what is currently is being dispatched. This should simplify things a lot
for us, since it makes it unnecessary to pass pointers through the
original handlers through all functions when we process messages, which
might require authentication.