lavf/libssh: improve authentication

- Add authentication using keys
- Provide better message on fail
Signed-off-by: Lukasz Marek <lukasz.m.luki@gmail.com>

libavformat/libssh.c

@@ -91,11 +91,14 @@ static int libssh_open(URLContext *h, const char *url, int flags)
         goto fail;
     }
 
-    if (pass && ssh_userauth_password(s->session, NULL, pass) != SSH_AUTH_SUCCESS) {
-        av_log(h, AV_LOG_ERROR, "Error authenticating with password: %s\n", ssh_get_error(s->session));
+    if (ssh_userauth_autopubkey(s->session, pass) != SSH_AUTH_SUCCESS) {
+        av_log(s, AV_LOG_DEBUG, "Authentication using public key failed, trying password method.\n");
+        if (ssh_userauth_password(s->session, NULL, pass) != SSH_AUTH_SUCCESS) {
+            av_log(h, AV_LOG_ERROR, "Authentication failed.\n");
             ret = AVERROR(EACCES);
             goto fail;
         }
+    }
 
     if (!(s->sftp = sftp_new(s->session))) {
         av_log(h, AV_LOG_ERROR, "SFTP session creation failed: %s\n", ssh_get_error(s->session));