url: provide dead_connection flag in Curl_handler::disconnect

It helps to prevent a hangup with some FTP servers in case idle session
timeout has exceeded.  But it may be useful also for other protocols
that send any quit message on disconnect.  Currently used by FTP, POP3,
IMAP and SMTP.

lib/transfer.c

@@ -1959,7 +1959,7 @@ connect_host(struct SessionHandle *data,
       res = Curl_async_resolved(*conn, &protocol_done);
     else
       /* if we can't resolve, we kill this "connection" now */
-      (void)Curl_disconnect(*conn);
+      (void)Curl_disconnect(*conn, /* dead_connection */ FALSE);
   }
 
   return res;