Bump x11-apps/lightdm from 1.7.9 to 1.8.2

- Add polkit-1 rules-file from fedora to allow lightdm to properly use
  systemd-logind, handle power management through upower and access to
  the shutdown/reboot menu.
- Update the systemd unit file for lightdm with up-to-date properties.
- Add tmpdfiles.d controlfile for creating /run/lightdm during boot with
  correct permissions.
- Purge lightdm-1.5.2-r1
Bump lightdm-gtk-greeter 1.6.0 -> 1.6.1