On Debian < 10:
- debhelper extends postint with 'try-restart', which caused re6st to
  remain stopped after upgrading from a re6st package that stopped
  services unconditionally in prerm (see previous commit);
- deb-systemd-invoke does not check whether the service is enabled
  so 'try-restart' can't be replaced blindly with 'restart'
  (as it is the case in recent Debian).