Default systemd units' stopIfChanged to a smarter value

[antifuchs]

Unless the unit has an ExecStop or ExecStopPost command defined, it could very easily "just" be restarted by systemd when switching configurations.

Thus, default stopIfChanged to false if the unit doesn't define either of these commands.

This change is the logical continuation of #359724, which was meant to prevent unnecessary stops/starts of systemd-udevd - it still does get restarted because the unit's stopIfChanged defaults to true, but we should probably set this to false for any unit that doesn't need it - restarting things is usually faster and disrupts service less; and sometimes it's even more correct than stopping/starting them, as in the case of udevd.

Things done

• Built on platform(s)
  • x86_64-linux
  • aarch64-linux
  • x86_64-darwin
  • aarch64-darwin
• For non-Linux: Is sandboxing enabled in nix.conf? (See Nix manual)
  • sandbox = relaxed
  • sandbox = true
• Tested, as applicable:
  • NixOS test(s) (look inside nixos/tests)
  • and/or package tests
  • or, for functions and "core" functionality, tests in lib/tests or pkgs/test
  • made sure NixOS tests are linked to the relevant packages
• Tested compilation of all packages that depend on this change using nix-shell -p nixpkgs-review --run "nixpkgs-review rev HEAD". Note: all changes have to be committed, also see nixpkgs-review usage
• Tested basic functionality of all binary files (usually in ./result/bin/)
• 25.05 Release Notes (or backporting 24.11 and 25.05 Release notes)
  • (Package updates) Added a release notes entry if the change is major or breaking
  • (Module updates) Added a release notes entry if the change is significant
  • (Module addition) Added a release notes entry if adding a new NixOS module
• Fits CONTRIBUTING.md.

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[antifuchs]

Sounds like this isn't going to be viable, as knowing if ExecStop is set for a unit isn't in the books for stuff like systemd.packages. Closing this, and setting stopIfChanged for the units that I know need it, then we can see about adding this to stc instead.