logind: when a bus call is done on a session, user or seat, optionally determine...

logind: when a bus call is done on a session, user or seat, optionally determine them from the caller credentials

More specifically, if an operation is requested on a session with an
empty name, the caller's session is used. If an operation is requested
on a seat with an empty name, the seat of the caller's session is used.
Finally, if an operation on the user with UID -1 is requested, the user
of the client's session is used (and not the UID of the client!).