Support for systemd-related behaviors

[Still34]

Summary

This PR aims to close #1041 by adding three new rules that address the behaviors of the sample mentioned in the issue.

• nursery/clear-system-logs-via-journalctl.yml
• nursery/interact-with-systemd-journal-via-journalctl.yml
• nursery/manage-system-services-via-systemctl.yml

The rules are currently in nursery due to the following concerns, and additional discussion surrounding them are welcome,

• Linux rules aren't very prevalent in capa in general; some additional testing with more ITW samples might be preferred.
• The current implementations use regex - not sure if there are any rules or performance concerns regarding regex usage.
• The scopes are currently set to file instead of function due to the targeted sample being Go-based; function-based scoping seems to break the detection.

[mike-hunhoff]

Thank you @Still34 . Please ensure the example file has been added to capa-testfiles and move the rules from the nursery to their respective folders.

[Still34]

I want to double check the following before moving the files to their respective folder. Let me know if all of these are cleared of concerns.

The rules are currently in nursery due to the following concerns, and additional discussion surrounding them are welcome,

* Linux rules aren't very prevalent in capa in general; some additional testing with more ITW samples might be preferred.

* The current implementations use regex - not sure if there are any rules or performance concerns regarding regex usage.

* The scopes are currently set to `file` instead of `function` due to the targeted sample being Go-based; `function`-based scoping seems to break the detection.