Add missing 'always' value support to periodic parameters (every n-days)

[Hufschmidt]

Pull Request (PR) description

This does not seem to be documented in the /etc/apt.conf.d/10periodic config comments, but according to the implementation of /usr/lib/apt/apt.systemd.daily (which is called by apt-daily.service and apt-daily-upgrade.service) some values also support 'always' in addition to positive integer value. (This affects settings that check an operation is only executed 'every n-days'.)

This Pull Request (PR) fixes the following issues

This change allows 'always' to be used for the following settings that support it and are managed by this module

• APT::Periodic::Update-Package-Lists
• APT::Periodic::Download-Upgradeable-Packages
• APT::Periodic::Unattended-Upgrade
• APT::Periodic::AutocleanInterval

Without this it is not possible to let the apt-daily.timer and apt-daily-upgrade.timer trigger multiple times a day since /usr/lib/apt/apt.systemd.daily will check its last run timestamp against the value of the above settings and abort.

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Sidenote: The following settings also support 'always' but are not managed by this module:

• APT::Periodic::CleanInterval
• APT::Periodic::BackupArchiveInterval

[Hufschmidt]

Good idea, added information to README.md.