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
8cdc8687bf