Enh: add a variant of DKIM-ID with conditional manual List-ID appended

[sbp]

The DKIM-ID generator of PR #517 does not include a manual List-ID. A version of that generator which includes the manual List-ID, but only if the same List-ID does not appear in the email, was suggested and seconded in that thread.

This issue is being created to provide a consolidated place of discussion for that mechanism, leaving the PR #517 thread to be about the DKIM-ID generator of that PR alone.

[sbp]

This enhancement must not be accepted. It creates a dangerous illusion that the users will be responsible for backing up data that should instead be stored using a reliable backup strategy. Moreover, it makes DKIM-IDs unnecessarily long, negating one of the primary purposes of DKIM-IDs. The other primary purpose, deduplication, would also be stymied by this malfeature.

[sebbASF]

Sorry, but I am not clear what you mean by a manual List-Id.
There does not seem to be a corresponding PR which might clarify matters.

Please explain.