linger-users: default to null, be explicit about null = imperative (#460311)

  - In all other cases, you'll need to set this option to `true` yourself.

  - `boot.isNspawnContainer` being `true` implies [](#opt-boot.isContainer) being `true`.

- `users.users.*.linger` now defaults to `null` rather than `false`, meaning NixOS will not attempt to enable or disable lingering for that user account.  In practice, this is unlikely to make a difference for most people, as new users are created without lingering configured, but it means users who use `loginctl` commands to manage lingering imperatively will not have their changes overridden by default.  There is a new, related option, `users.manageLingering`, which can be used to prevent NixOS attempting to manage lingering entirely.

- `users.users.*.linger` now defaults to `null` rather than `false`, meaning NixOS will not attempt to enable or disable lingering for that user account, instead allowing for imperative control over lingering using the `loginctl` commands. In practice, this is unlikely to make a difference for most people, as new users are created without lingering configured. There is a new, related option, `users.manageLingering`, which can be used to prevent NixOS attempting to manage lingering entirely.

- Due to [deprecation of gnome-session X11 support](https://blogs.gnome.org/alatiera/2025/06/08/the-x11-session-removal/), `services.desktopManager.pantheon` now defaults to pantheon-wayland session. The X11 session has been removed, see [this issue](https://github.com/elementary/session-settings/issues/91) for details.

  '';

  userOpts =

    let

      # Pass state version through despite config being overwritten in the inner module

      inherit (config.system) stateVersion;

    in

    { name, config, ... }:

    {

        linger = mkOption {

          type = types.nullOr types.bool;

          example = true;

          default = if versionOlder stateVersion "26.11" then false else null;

          defaultText = literalExpression "if lib.versionOlder config.system.stateVersion \"25.11\" then false else null";

          default = null;

          description = ''

            Whether to enable or disable lingering for this user.  Without

            lingering, user units will not be started until the user logs in,

            `logind.conf`.

            By default, NixOS will not manage lingering, new users will default

            to not lingering, and you can change the linger setting using

            to not lingering, and lingering can be configured imperatively using

            `loginctl enable-linger` or `loginctl disable-linger`. Setting

            this option to `true` or `false` is the declarative equivalent of

            running `loginctl enable-linger` or `loginctl disable-linger`