The core can be gracefully shutdown by sending a SIGINT to its process.
However, this signal is also propagated to all child processes, i.e. the
forked ffmpeg processes. They will also be stopped and might reconnect.
This is not wanted. The core has to stop these processes.
The child process will now get their own session ID with setsid() before
replacing themselves with ffmpeg. This way they will not receive a SIGINT
that was meant only for the parent.
If a process has some cleanup with purge-on-delete defined, the purge
has to wait until the process actually exited. Otherwise it may happen
that the process got the signal, files are purged, but the process is
still writing some files in order to exit cleanly. This would lead to
some artefacts left on the filesystem.
