Added Slackware Linux support

[giterlizzi]

• Detect Slackware Linux via lsb_release or via /etc/slackware-version file
• Use slackpkg for download, install, updare and remove packages
• Use /etc/rc.d/rc.* files for start & stop Slackware Linux services

Checklist

• [x] based on top of latest source code
• [x] changelog entry included
• [ ] tests pass in CI
• [x] git history is clean
• [ ] git commit messages are well-written

[ferki]

Thanks for your contribution, @giterlizzi, and for you patience!

This PR does not fully follow the project's expectations, so I'd like to take this chance to kindly link our Contributing guide, specifically the section about GitHub issues and pull requests.

I would have preferred to explain the below bits in an issue first, but since this PR is already here, we will do that next time :)

---

As far as adding support for new operating systems is concerned, it would be best to do that as an extension module instead of here in core (see also Rex core vs extending Rex).

I feel it would not be responsible on my part to merge the whole support when I don't have the necessary knowledge nor resources to review the code or to commit to long-term maintaining it (in fact, I would very much like to split out the already included similar modules too).

Instead, I would be happy to help you bring Slackware support to the community as an extension, and to merge any "glue code" to core which is needed for that to work. To discuss any details and questions, please reach out on one of our community support channels, or even book an appointment directly with me for my open source office hours.

---

Conceptionally speaking, all the major components for a new OS support are present here, so the code is probably in a good shape already. The parts we'd need to add to core are the first two steps in the section linked above (recognizing Slackware, and choosing the right Pkg and Service manager modules when Slackware is detected).

Please note there was a previous attempt to add Slackware support via #1387, which was sadly abandoned, but some parts of it might still serve you great as inspiration.

Currently CI checks indicate that "Some checks were not successful", so I did not do a full review yet. Let me know if you still would like a full review before going ahead.

I look forward to working with you on Slackware support!