zincati.service: periodically restart zincati daemon

We've seen some issues in the past where a simple restart of the
zincati daemon would have allowed systems to continue updating.
Let's periodically restart the zincati daemon to handle cases
like this in the future, which we can't always foresee.

The most recent example being: coreos/fedora-coreos-tracker#1608

dist/systemd/system/zincati.service

@@ -22,6 +22,7 @@ Type=notify
 ExecStart=/usr/libexec/zincati agent ${ZINCATI_VERBOSITY}
 Restart=on-failure
 RestartSec=10s
+RuntimeMaxSec=3w
 
 [Install]
 WantedBy=multi-user.target