ssh_terminal: flush before closing connection

Originally the outgoing buffer was flush when the terminal was
detached. Since we cannot do that at this point, make the flushing
part of the common cleanup procedure.

Issue #3682.

repos/gems/src/server/ssh_terminal/server.cc

@@ -193,6 +193,7 @@ void Ssh::Server::_cleanup_session(Session &s)
 	ssh_channel_free(s.channel);
 	s.channel = nullptr;
 
+	ssh_blocking_flush(s.session, 5*1000);
 	ssh_event_remove_session(_event_loop, s.session);
 	ssh_disconnect(s.session);
 	ssh_free(s.session);