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.