Enable Salsa-CI and fix issues that were immediately made apparent by CI

Motivation for these changes

Enable Salsa-CI and fix several failures to make it builds and Lintian pass.

Having Salsa-CI enabled will help ensure easily machine detectable regressions don't slip back into the code base later.

This also makes any future contribution process faster and more reliable, as any contributor submitting a Merge Request will get immediate feedback, and the maintainers save time by not having to point out basic mistakes.

I am intentionally submitting multiple commits in this MR as the first one enables testing and all the follow-up ones fix issues that testing revealed. The commit from @diederik is included with his permission (the original MR !16 (closed) was already closed due to lack of review for a very long time).

Intentionally no changelog

These commits have intentionally no debian/changelog updates as it causes every single rebase or cherry-pick of a commit to always have a merge conflict. It is much better to have all commits as-is, and then right before upload just run gbp-dch --auto to automatically generate the changelog.

Details

For details, see each commit message.

How to review

I recommend reviewing this by opening the "Commits" tab and clicking on the bottom one commit in the list (the first commit), and then reading each commit, moving to next one by pressing "Next". On first comment press Start review and after reading last commit press Submit review. Screenshot below exemplifies where you should be clicking in case GitLab is not familiar to the reviewer.

How to merge

Each commit message is self-standing and meticously documented. Please do not squash or rewrite, but merge directly as-is. I am happy to rebase/polish as much as the reviewers request.