Wait using a condition variable in server

Currently, server_stop() is called from the same thread as
server_connect_to(), so interruption may never happen.

This is a step to prepare executing the server from a dedicated thread.

app/src/server.h

@@ -47,6 +47,10 @@ struct server {
     // alive only between start() and stop()
     struct sc_process_observer observer;
 
+    sc_mutex mutex;
+    sc_cond cond_stopped;
+    bool stopped;
+
     sc_socket server_socket; // only used if !tunnel_forward
     sc_socket video_socket;
     sc_socket control_socket;