nixos/kresd: fix systemd dependency cycle

[erosennin]

Motivation for this change

kresd.service makes sockets.target depend on itself causing a dependency cycle:

$ systemd-analyze verify default.target
sockets.target: Found ordering cycle on sockets.target/start
sockets.target: Found dependency on kresd.service/start
sockets.target: Found dependency on kresd-cachedir.service/start
sockets.target: Found dependency on basic.target/start
sockets.target: Found dependency on sockets.target/start
sockets.target: Breaking ordering cycle by deleting job kresd.service/start
kresd.service: Job kresd.service/start deleted to break ordering cycle starting with sockets.target/start

As a result, kresd.service won't start on system boot. This PR fixes that.

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • NixOS
  • macOS
  • other Linux distributions
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

---

[vcunat]

❤️
The service is the same as 17.09, and I think the problem doesn't happen. I'll try to look deeper tomorrow.

[vcunat]

Hmm, I get the warning but the service always starts anyway.

[vcunat]

🆗 Added a few other improvements. Please, @mention me for anything related to knot-resolver. Feedback is welcome.

[erosennin]

Thanks for the fast response!

[jbboehr]

Is this going to be backported to 17.09?

[vcunat]

Right, done.

[jbboehr]

Thanks!