Support systemd drop files

[twiggler]

Fixes:

• _select_parser could be invoked with a Path, while it expects a FileSystemEntry. Additionally, there were multiple code paths in parse and parse_config to support TargetPath, Path, and File objects. Removed the dependence on FileSystemEntry by converting entries to TargetPath in the config file system.
• Systemd network parser did not collapse singleton values
• When multiple sections with the same name are encountered, the resulting key is a list of those sections:
  Example:

key value1
   key2 value2a
key value1
   key2 value2b
   key3 value3b

now parses as {"key value1": [{"key2": "value2a"}, {"key2": "value2b", "key3": "value3b"}]}. Previously, this parsed as {"key value1": {"key2": ["value2a", "value2b"], "key3": "value3b"}}, where the parallel lists could become misaligned.

Supports:

• Drop files when invoking parse with a systemd file
• Empty value now clears corresponding key when merging systemd files.

[codecov]

Codecov Report

Attention: Patch coverage is 95.23810% with 3 lines in your changes missing coverage. Please review.

Project coverage is 77.85%. Comparing base (e56c684) to head (50f3a87).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
dissect/target/helpers/configutil.py	94.54%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1136      +/-   ##
==========================================
- Coverage   77.91%   77.85%   -0.06%     
==========================================
  Files         359      359              
  Lines       32665    32701      +36     
==========================================
+ Hits        25450    25460      +10     
- Misses       7215     7241      +26     

Flag	Coverage Δ
unittests	77.85% <95.23%> (-0.06%)	⬇️

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[codspeed-hq]

CodSpeed Performance Report

Merging #1136 will not alter performance

_{Comparing systemd-drop (50f3a87) with main (e56c684)}

Summary

✅ 3 untouched benchmarks