Plugins: Improve c plugin (doc, tests, refactor)

[flo91]

• some small refactoring (e.g. remove unnecessary dups, reduce scope of variables)
• add code documentation (Doxygen)
• extended README.md
• add unit tests

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  (added as entry in doc/news/_preparation_next_release.md which contains _(my name)_)
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  • add a line in doc/news/_preparation_next_release.md
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  • fix all memleaks
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Checklist

• I added unit tests for my code
• I fully described what my PR does in the documentation
  (not in the PR description)
• I fixed all affected documentation (see Documentation Guidelines)
• I fixed all affected decisions (see Decision Process)
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• Documentation is introductory, concise, good to read and describes everything what the PR does
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[atmaxinger]

In what state is this? Do you think this should also be part of the v0.11 release?

[flo91]

This PR is basically finished and ready for reviews.
If the reviewers approve the changes, I don't see a reason why we shouldn't include it in v0.11.

[markus2330]

Great, yes, let us take this in. Shell recorder tests would have been nice.

[markus2330]

one line per sentence according to documentation guidelines.

[atmaxinger]

Great work!

[atmaxinger]

jenkins build libelektra please

[atmaxinger]

jenkins build libelektra please

[atmaxinger]

jenkins build libelektra please