Add ssh_userauth_publickey_auto_get_current_identity

We would like to use ssh_userauth_publickey_auto in the ssh client parts of the Cockpit web console, so that the keys we try when logging into a remote host are the same as everyone else is trying. I think that ssh_userauth_publickey_auto is our best shot at avoiding mysterious behavior in Cockpit, just because we have our own code for doing public key auth.

Unfortunately, I haven't found a way to make it work for us: We need to know which identity needs to be unlocked with a passphrase, and there doesn't seem to be a API for that.

See https://github.com/cockpit-project/cockpit/pull/14551 for my trials and tribulations.

So I propose to add this function which we can then call from the auth_function callback.

Thanks for your consideration!

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