Select appropriate systemd service based on your compile settings. If you
enabled WITH_SYSTEMD, use mosquitto.service.notify, otherwise use
mosquitto.service.simple. The service must be renamed to mosquitto.service
before usage. Don't forget to change default paths in service file if you
changed the default build settings.

With WITH_SYSTEMD mosquitto will notify a complete startup after
initialization. This means that follow-up units can be started after full
initialization of mosquitto (i.e. sockets are opened).