minor improvements for debugging

* Info page: status of Serial (can be disconnected on new MCUs), and owner of RX/TX pins
* improved handling of Serial (don't write if not connected)
* added min available stack size to debug output (esp32 only)
* webserver show remaining stack size
* webserver: show string buffer usage

wled00/ws.cpp

@@ -117,6 +117,8 @@ void sendDataWs(AsyncWebSocketClient * client)
     DEBUG_PRINTLN(F("Out of memory (WS)!"));
     return;
   }
+  #else
+    //DEBUG_PRINT(pcTaskGetTaskName(NULL)); DEBUG_PRINT(F(" min free stack ")); DEBUG_PRINT(uxTaskGetStackHighWaterMark(NULL));
   #endif
   buffer = ws.makeBuffer(len); // will not allocate correct memory sometimes on ESP8266
   #ifdef ESP8266