upgrades: dist-upgrade can be interrupted

• Version: 21.4.2~bpo10+1
• Hardware: Pioneer kit

Tested by manually running action ./upgrades start-dist-upgrade --test.

My guess is that the upgrades led to a system reboot, which interrupted the dist-upgrade process.

However by running ./upgrades start-dist-upgrade again, it was able to complete the process. (This command is run once per day, so it should recover by the next day.)

One issue at the end: The apt hold was left on the freedombox package. I don't really know why. (see edit below) We may need to just detect and recover from this situation.

EDIT: plinth.action_utils.apt_hold will detect if there is already a hold on freedombox. But in this case it does not attempt to remove the hold.

Tasks:

• Diagnostic to detect and repair package holds.
• Diagnostic to detect and repair leftover dist-upgrade flag.
• Inhibit shutdown during dist-upgrade.