calling functions to support that with the following API notes:
* libssh2_publickey_shutdown(), libssh2_session_free() changed
to return an "int" to allow signaling of LIBSSH2_ERROR_EAGAIN.
* libssh2_scp_recv(), libssh2_scp_send_ex() and libssh2_sftp_init()
will loop in on libssh2_channel_free() when there is an error.
It is not possible to return LIBSSH2_ERROR_EAGAIN in this condition
in these 3 functions and not lose the original error code.
non-blocking mode. All functions either return
LIBSSH2_ERROR_EAGAIN or return NULL and set the error code
to be LIBSSH2_ERROR_EAGAIN instead of blocking, when in
non-blocking mode.
