Add --root to the etc plugin

[Miauwkeru]

• Fix MdVolumeSystem
• Add root to etc plugin, look for entries relative to root
• Make it consistent to use a configuration entry
• Made the directory lookup work relatively

[codecov]

Codecov Report

Attention: Patch coverage is 77.77778% with 6 lines in your changes missing coverage. Please review.

Project coverage is 75.26%. Comparing base (5d19f13) to head (71c8e15).

Files	Patch %	Lines
dissect/target/filesystems/config.py	86.36%	3 Missing ⚠️
dissect/target/plugins/os/unix/etc/etc.py	50.00%	2 Missing ⚠️
dissect/target/helpers/configutil.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #783   +/-   ##
=======================================
  Coverage   75.26%   75.26%           
=======================================
  Files         296      296           
  Lines       25575    25584    +9     
=======================================
+ Hits        19248    19257    +9     
  Misses       6327     6327           

Flag	Coverage Δ
unittests	75.26% <77.77%> (+<0.01%)	⬆️

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