updater: Adding --fix-broken stages

The updater will now try to fix broken packages before and after
dist-upgrade. This can help in some cases when there's something
hanging not configured or just part-installed.

Fixes https://github.com/KanoComputing/peldins/issues/721

Tested locally on the pi, dr build works fine.

Also notice, that I describe the stages very vaguely (instead of 'Fixing broken packages' which sounds kind-of bad, when normally there won't be anything wrong).

cc @alex5imon