Support for libssh2_channel_request_auth_agent in with-ssh-connection macro

[svetlyak40wt]

When :forward-agent t argument is passed to with-ssh-connection macro,
it will use libssh2_channel_request_auth_agent call before execution the body.

The function was merged on 13 August 2019
and should be included in the libssh2 > 1.9.0 which is not released yet.

Warning

This code works without errors for me, but agent is not forwarded.

Steps to reproduce:

1. Install fresh libssh2 from the GitHub: brew install --build-from-source libssh2 --HEAD
2. Run this code:

(defun test-github (host)
           (flet ((-> (from to)
                    (loop for line = (read-line from nil nil)
                          while line
                          do (write-string line to)
                             (terpri))))
             (ssh:with-connection (conn host (ssh:agent "root"))
               (ssh:with-command (conn stream "ssh -T git@github.com")
                                 (-> stream *standard-output*)))))


(test-github "some-host-where-i-have-access")

Expected output is:

Hi svetlyak40wt! You've successfully authenticated, but GitHub does not provide shell access.

Actual output:

git@github.com: Permission denied (publickey).

Final note

I wasn't able to compile original agent forwarding example from libssh2 to check if the C library works as expected.

Maybe somebody can help me with that?