Bugfix 458824 and 438001

Bugs [458824](https://bugs.kde.org/show_bug.cgi?id=458824) and [438001](https://bugs.kde.org/show_bug.cgi?id=438001) are closely related, this MR will deal with both.

changes:

- Moved systemdjob to core from `firewalld` backend. Now both backends (`firewalld` and `ufw`) services are enabled/disabled together with the firewall starting process.
- Don't let linux specifics firewall backends build on BSD.