Fix the screendump service.

Going back to the configuration of my first tests.

I suppose the sdump user must be below 1000, so that systemd will not
launch a full user session for us (with pulse & friends).

As I'm hiding the PIDs, I don't need to run the ffmpeg process with
a different name. I'm not sure if pkill was able to find the process
with the changed name, so we go back to a simpler script.

And it works!