add a stable sort function

Douglas Bagnallrequested to merge
samba-team/devel/samba:douglas-stable-sort into master

For some work on lzxpress Huffman encoding, and for some old bugs on BSDs, we really want to have a reliable stable sort algorithm.

As per the commit message, GNU qsort() is almost always stable, but we can neither completely rely on that, nor leave those other people out in the cold.

Checklist

  • Commits have Signed-off-by: with name/author being identical to the commit author
  • (optional) This MR is just one part towards a larger feature.
  • (optional, if backport required) Bugzilla bug filed and BUG: tag added
  • Test suite updated with functionality tests
  • Test suite updated with negative tests
  • Documentation updated
  • CI timeout is 3h or higher (see Settings/CICD/General pipelines/ Timeout)

Reviewer's checklist:

  • There is a test suite reasonably covering new functionality or modifications
  • Function naming, parameters, return values, types, etc., are consistent and according to README.Coding.md
  • This feature/change has adequate documentation added
  • No obvious mistakes in the code

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