[debian] Systemd debug messages

[tmagik]

From the forums..

I’m using the system via screen so the constant messages on the console make it
borderline unusable. But if the debug flag is helpful for other messages, I tried adding
the ‘systemd.log_level=info’ argument per this thread [1] and that works well - it’s just
the systemd debug messages which interfere.

[1] https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=902795