PolkitSystemBusName: Retrieve both pid and uid
For polkit_system_bus_name_get_process_sync(), as pointed out by Miloslav Trmac, we can securely retrieve the owner uid as well from the system bus, rather than (racily) looking it up internally. This avoids use of a deprecated API. However, this is not a security fix because nothing in the polkit codebase itself actually retrieves the uid from the result of this API call. But, it might be useful in the future.