nixos/ntpd: Use StateDirectory instead of a preStart script

  cfg = config.services.ntp;

  stateDir = "/var/lib/ntp";

  configFile = pkgs.writeText "ntp.conf" ''

    driftfile ${stateDir}/ntp.drift

    driftfile /var/lib/ntp/ntp.drift

    restrict default ${toString cfg.restrictDefault}

    restrict -6 default ${toString cfg.restrictDefault}

      isSystemUser = true;

      group = "ntp";

      description = "NTP daemon user";

      home = stateDir;

      home = "/var/lib/ntp";

    };

    users.groups.ntp = { };

      wants = [ "time-sync.target" ];

      before = [ "time-sync.target" ];

      preStart = ''

        mkdir -m 0755 -p ${stateDir}

        chown ntp ${stateDir}

      '';

      serviceConfig = {

        ExecStart = "@${ntp}/bin/ntpd ntpd -g ${builtins.toString ntpFlags}";

        Type = "forking";

        StateDirectory = "ntp";

      };

    };