xfce: add screenLock option

[zarelit]

Motivation for this change

screenLock option is needed to provide at least one application for xflock4 to lock the screen.

Closes #16845

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 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]

@zarelit, thanks for your PR! By analyzing the history of the files in this pull request, we identified @edolstra, @abbradar and @peti to be potential reviewers.

[grahamc]

Thank you!

[Baughn]

This seems to have caused a fun new bug in my system.

Basically, if you have xlockmore installed in the user environment -- as you would, to make xfce screen-lock work -- then, with this change, the screensaver becomes unable to unlock itself. I have no idea how this could possibly be, but it's fixed by nix-env -e xlockmore.

My first suspicion was pam, as the pam input is optional in the xlockmore recipe, but both of the xlockmore binaries on my system link against pam.

[Baughn]

Some stracing found the problem: For whatever reason, with xlockmore installed in my user profile, it's calling unix_chkpwd for root instead of, well, me.

I'm really quite terribly confused now.

[zarelit]

@Baughn Did you do further investigation or just removed xlockmore from your user profile?

[Baughn]

@zarelit I removed xlockmore from my user profile.