Comment 26 for bug 1690980

IMO the proper fix involves several changes which I collected in this GitHub branch to be merged
to upstream:
https://github.com/rbalint/unattended-upgrades/commits/lock-handling

Since the changes are extensive I would prefer merging them upstream, releasing them to sid, then Artful, and then they can be backported to Xenial and other releases. It looks like to me that more code needs to be backported than half of the delta between releases thus I propose SRU-s practically syncing latest unattended-updates to supported releases.

Testing of the new code is appreciated, but please let u-u be triggered by the apt-daily* timers because sudo does not protect u-u from being killed as Ethan pointed out in #25.