Way for traced & traced_probes systemd services to signal readiness

As part of debianizing, it would be nice to have a way for traced and traced_probes systemd services to signal when they are ready to process requests.

We can consider to:
1) introduce a flag called is_systemd (which can also be used for different logging purposes)
2) Depending on the flag use [sd_notify](https://man7.org/linux/man-pages/man3/sd_notify.3.html)'s standalone implementation to send sd_notify(0, "READY=1") to indicate we are active.
3) Update debian .service files to change type to notify

Open to other simpler proposals