nixos/systemd: Refactor references to `config.systemd`

      "systemd/system.conf".text = ''

        [Manager]

        ManagerEnvironment=${lib.concatStringsSep " " (lib.mapAttrsToList (n: v: "${n}=${lib.escapeShellArg v}") cfg.managerEnvironment)}

        ${optionalString config.systemd.enableCgroupAccounting ''

        ${optionalString cfg.enableCgroupAccounting ''

          DefaultCPUAccounting=yes

          DefaultIOAccounting=yes

          DefaultBlockIOAccounting=yes

          DefaultIPAccounting=yes

        ''}

        DefaultLimitCORE=infinity

        ${optionalString (config.systemd.watchdog.device != null) ''

          WatchdogDevice=${config.systemd.watchdog.device}

        ${optionalString (cfg.watchdog.device != null) ''

          WatchdogDevice=${cfg.watchdog.device}

        ''}

        ${optionalString (config.systemd.watchdog.runtimeTime != null) ''

          RuntimeWatchdogSec=${config.systemd.watchdog.runtimeTime}

        ${optionalString (cfg.watchdog.runtimeTime != null) ''

          RuntimeWatchdogSec=${cfg.watchdog.runtimeTime}

        ''}

        ${optionalString (config.systemd.watchdog.rebootTime != null) ''

          RebootWatchdogSec=${config.systemd.watchdog.rebootTime}

        ${optionalString (cfg.watchdog.rebootTime != null) ''

          RebootWatchdogSec=${cfg.watchdog.rebootTime}

        ''}

        ${optionalString (config.systemd.watchdog.kexecTime != null) ''

          KExecWatchdogSec=${config.systemd.watchdog.kexecTime}

        ${optionalString (cfg.watchdog.kexecTime != null) ''

          KExecWatchdogSec=${cfg.watchdog.kexecTime}

        ''}

        ${config.systemd.extraConfig}

        ${cfg.extraConfig}

      '';

      "systemd/sleep.conf".text = ''

        [Sleep]

        ${config.systemd.sleep.extraConfig}

        ${cfg.sleep.extraConfig}

      '';

      "systemd/system-generators" = { source = hooks "generators" cfg.generators; };