Merge branch 'ipv4_fallback' of https://gitlab.com/quobert/Simple-Web-Server

      if(!acceptor)

        acceptor = std::unique_ptr<asio::ip::tcp::acceptor>(new asio::ip::tcp::acceptor(*io_service));

      try {

        acceptor->open(endpoint.protocol());

      } catch (const system_error& error) {

        if(error.code() == asio::error::address_family_not_supported && config.address.empty()) {

          endpoint = asio::ip::tcp::endpoint(asio::ip::tcp::v4(), config.port);

          acceptor->open(endpoint.protocol());

        } else {

          throw;

        }

      }

      acceptor->set_option(asio::socket_base::reuse_address(config.reuse_address));

      if(config.fast_open) {

#if defined(__linux__) && defined(TCP_FASTOPEN)