Way for traced & traced_probes systemd services to signal readiness

[sashwinbalaji]

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'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