Soften behaviour of the Compression=no/yes option

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