Add back Cinnamon polkit authentication agent

The Cinnamon polkit authentication agent is still used by Arch Linux and (I guess) by Fedora, so please keep it. I prefer this one over polkit-gnome.

This change reverts the following commits partially:
https://github.com/linuxmint/Cinnamon/commit/4873e7f4c5b53b5cc6dc3149d1165a7c84bc3fc0
https://github.com/linuxmint/Cinnamon/commit/72b406cc39e7571cd7d22ee132c2472617afddfe
https://github.com/linuxmint/Cinnamon/commit/b8d1dbba7cc2a6edc2bc429b82518356ff73bf6d