Allow to always listen to specific addresses for an accessible data flow.

[dantelex]

"always listen" to the person that sent it means, "I don't want ever manually to start a receiving session apart from the first-time configuration. I want to specify port 78089, and scs should always listen on that port".

We will need to add scs as a system service. When the machine starts up, immediately start scs too. When it is about to shut down, close it too. A systemd service on *ix machines.