Improve Linux services parsing

[JSCU-CNI]

This PR improves Linux services parsing by switching to the configutil parser instead of using the etc plugin. We now also check if if a segment contains any configuration before attempting to parse it.

[codecov]

Codecov Report

Attention: Patch coverage is 83.33333% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.45%. Comparing base (092e4f9) to head (c7d627d).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
dissect/target/plugins/os/unix/linux/services.py	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1049      +/-   ##
==========================================
- Coverage   78.46%   78.45%   -0.01%     
==========================================
  Files         337      337              
  Lines       29698    29701       +3     
==========================================
  Hits        23303    23303              
- Misses       6395     6398       +3     

Flag	Coverage Δ
unittests	78.45% <83.33%> (-0.01%)	⬇️

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[Miauwkeru]

Technically, the plugin never used the etc plugin. As it used the config_tree which was bound on / instead of /etc.

I do agree it's a lot better to just immediately use the configutil.parse instead. As it is more direct and only requires a path.