Soften behaviour of the Compression=no/yes option

Fabian Vogtrequested to merge
Vogtinator/libssh:compdefault into master

Currently Compression=no (the default) force-disables zlib algos, while Compression=yes force-enables it. This means that mismatching options between client and server lead to connection failure. This can easily happen if the server has default settings but the client specifies Compression=yes.

OpenSSH treats the option as a "prefer compression" setting:

Compression=no -> none,zlib@openssh.com,zlib (default)
Compression=yes -> zlib@openssh.com,zlib,none

This commit changes the libssh behaviour to the same as OpenSSH.

Checklist

  • Commits have Signed-off-by: with name/author being identical to the commit author
  • Code modified for feature
  • Test suite updated with functionality tests
  • Test suite updated with negative tests
  • Documentation updated

Reviewer's checklist:

  • Any issues marked for closing are addressed
  • There is a test suite reasonably covering new functionality or modifications
  • Function naming, parameters, return values, types, etc., are consistent and according to CONTRIBUTING.md
  • This feature/change has adequate documentation added
  • No obvious mistakes in the code
Edited by Fabian Vogt

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