Add ssh_send_issue_banner() API

Add ssh_send_issue_banner() API for sending the SSH server's banner.

The current libssh-server cannot send the banner(ex. /etc/issue.net). So I propose to add ssh_send_issue_banner() API.

If ssh_send_issue_banner() called in the auth_none_function of ssh_server_callback_struct, you can send the SSH server banner to the SSH client.

I need this feature while using libssh.

Signed-off-by: Seung Min Park smpark@pnpsecure.com

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