Maintenance tasks of Debian servers.

- mnpw is now working (enough) so let's put it to use for all of the
  various Nagios-style checks.
- Add the monitoring plugins from monitoring-plugins.org/ for check_apt.
  Now I should get better alerts for pending updates. Regular updates
shuold be just a warning but security updates should be a critical
alert. Let's put it in production and see.
- Now with check_apt handling APT updates, write a small shell script
  for a Nagios-style check for firmware updates.
- And the last part of the updates, update flatpaks silently.
- Refactor of the general maintenance and btrfs-specific tasks.
- Tag the include_tasks tasks with always, so those tasks are always
  included so their tags are also evaluated.
- Remove the apt-transport-https package installation, it's a dummy
  package for apt since Debian Buster and Ubuntu Bionic.