spice-vdagent: add systemd support

[offlinehacker]

Motivation for this change

Support for systemd to spice-vdagent. This pull request should fix clipboard for spice.

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
  • Linux
• 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.

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[mention-bot]

@offlinehacker, thanks for your PR! By analyzing the history of the files in this pull request, we identified @aboseley and @grahamc to be potential reviewers.

[bjornfor]

Please add to the commit message what "systemd support" brings.

[offlinehacker]

I gave it a better description, but honestly i forgot what it fixed for me, but i assume, as systemd-logind is used on nixos, that clipboard did not work before this additional dependency, as it was not able to get user session information.