0.8.6

This is a minor update release.
Some changes include:

- fix flush_area fast-path
- Drop dead code
- seed random number generator by default for script plugins
- allow plymouth to be built without gtk dependency
- cope with lack of /etc/os-release better
- pull in systemd unit files
- add support for new "updates" mode for doing package updates
  (only available for some plugins at the moment)