Fix location of systemd service file on Debian systems

[vincentbernat]

Administrators are not expected to put files in /lib. The correct
location for the systemd service file is in
/etc/systemd/system (like for RHEL).

This is only a change in documentation. Also, I think we should not use SIGKILL as the kill. Is the issue still present in recent versions of oxidized? Just using SIGTERM works for me. Cc @mattie47

[codecov-io]

Codecov Report

Merging #1840 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1840   +/-   ##
=======================================
  Coverage   63.42%   63.42%           
=======================================
  Files          30       30           
  Lines        1490     1490           
=======================================
  Hits          945      945           
  Misses        545      545

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[mattie47]

@vincentbernat I'm not sure. I used oxidized for around a year on two different machines and the sigkill was always needed. Otherwise I always had to find the pid and kill -9 it.

I haven't used oxidized now in about a year so I don't know if anything has changed (in a different job).

[wk]

Pulling this as-is, since the recommendation to use /etc/systemd/system is consistent with distribution guidelines. If SIGKILL vs SIGTERM is relevant and should be fixed, don't hesitate to open a separate PR!

Thank you!