[WIP] communtiy/polkit: move from main
This MR contains the necessary changes to move polkit to community/. This allows us to move python2 to community/ as well in a later MR. Besides, I believe that community/ is the better place for polkit anyhow as most desktop-related packages are in community/.
Nonetheless, this MR needs some further discussion as main/libvirt is now build without polkit support. I don't know yet what this entails (i.e. if it breaks other aports). Any thoughts?