polkitd: Only exit successfully if we owned the bus name
It does not make sense to exit successfully just because the service entered the mainloop. If e.g. the system bus cannot be reached, then polkitd shouldn't return a success state.
This does usually not matter. However, it caused some headache for me within python-dbusmock, where my service would wait for polkitd activation which would never happen. But, only if the test was run as root.