portable way to clear tcp pcb in time-wait (esp8266/Arduino) - tcpCleanup() fix for lwip2

tcpCleanup-snippet.ino

// https://github.com/esp8266/Arduino/issues/1923
// https://github.com/esp8266/Arduino/issues/4213
// compatible with lwip-v1 and -v2
// no need for #include

struct tcp_pcb;
extern struct tcp_pcb* tcp_tw_pcbs;
extern "C" void tcp_abort (struct tcp_pcb* pcb);

void tcpCleanup ()
{
  while (tcp_tw_pcbs != NULL)
  {
    tcp_abort(tcp_tw_pcbs);
  }
}