Minimal Viable FSMPI Deployment

Minimal Viable FSMPI Deployment ... that allows updating mail.fsmpi and lists.fsmpi.

Reviewable, but should NOT be merged before successful deployment. Split into multiple commits to ease reviews.

Only commits with messages identifying fsmpi-specifics need to be refactored and removed after the upgrade. Other commits may introduce variables that are documented not to be relied upon in availability and functionality. However, we may decide to keep them (probably slightly repurposed) later on.

Note that postfix received some configuration cleanups. There are slight changes, but due to current defaults and main.cf <-> master.cf interaction the result should be sufficiently close in expected functionality. That being said, I'd passed by some further options to be introduced some time after the upgrade.