nixos/emacs: make systemd report clean exit status when stopping (#397916)

        serviceConfig = {

          Type = "notify";

          ExecStart = "${pkgs.runtimeShell} -c 'source ${config.system.build.setEnvironment}; exec ${cfg.package}/bin/emacs --fg-daemon'";

          ExecStop = "${cfg.package}/bin/emacsclient --eval (kill-emacs)";

          # Emacs exits with exit code 15 (SIGTERM), when stopped by systemd.

          SuccessExitStatus = 15;

          Restart = "always";

        };