Improve Linux OS detection

[Horofic]

Fixes #807

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.51%. Comparing base (95f27ae) to head (71e4a80).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #809   +/-   ##
=======================================
  Coverage   75.51%   75.51%           
=======================================
  Files         305      305           
  Lines       26348    26348           
=======================================
  Hits        19897    19897           
  Misses       6451     6451           

Flag	Coverage Δ
unittests	75.51% <100.00%> (ø)

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[Schamper]

What about this? This also finally fixes the problem that if you run the local loader with the incorrect permissions, it will wrongly select FreeBSD as the OS with /proc as the root filesystem because /proc/net exists.