Harden service file: Set OOMScoreAdjust to -1000

[Cropi]

From the systemd.exec(5) man page:

OOMScoreAdjust=
      Sets the adjustment value for the Linux kernel's Out-Of-Memory (OOM) killer score for
      executed processes. Takes an integer between -1000 (to disable OOM killing of processes of
      this unit) and 1000 (to make killing of processes of this unit under memory pressure very
      likely). 

The out-of-memory killer in the kernel can easily kill the usbguard-daemon for example when unauthorized process creates many small processes, which causes the OOM algorithm to see usbguard-daemon as the best candidate to be killed, instead of aiming
at some of those unprivileged processes. I have not seen an example of this in practice, but I think it would be good to include.

[hartwork]

Seems like a good idea 👍