redis: handle changes to systemd support

The 6.0 changelog notes that systemd support was rewritten. The effects
of that seem to be twofold:

* Redis will silently fail to sd_notify if not built with libsystemd,
  breaking our unit configuration.
* It also appears to misbehave if told to daemonize when running under
  systemd -- note that upstream's sample unit configuration does not
  daemonize:
  https://github.com/antirez/redis/blob/unstable/utils/systemd-redis_server.service