sftp: Cap maximum SFTP write

The curl is using the sftp write without any chunking, which breaks on some servers having a limit on how large SFTP packets they can accept.

The SFTP specification suggests the maximum SFTP packet size in https://datatracker.ietf.org/doc/html/draft-ietf-secsh-filexfer-02#section-3 and curl claims that this is not their issue https://github.com/curl/curl/pull/11804 so it looks like we will have to cap the maximum SFTP size inside of libssh.

There are different approaches proposed:

• simple caping and returning smaller than passed value is probably least intrusive
• cycling over the all passed data and sending it in separate packets can cause very short packets and more synchronization needed

I am open to other proposals.

Fixes: #215 (closed)

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