Allow faster reuse of socket, to be able to restart WifiServer. (#4306)

See #3960 for more details of the problem and the solution. I only implemented what was proposed in this ticket, as it solves my problem, which was the same as in this ticket. Credits for the code going to @etrinh ;-)

This also is a more consistence behaviour compared to esp8266, where it also is possible to restart the wifiserver immediately on the same port.

libraries/WiFi/src/WiFiServer.h

@@ -40,6 +40,7 @@ class WiFiServer : public Server {
     WiFiClient available();
     WiFiClient accept(){return available();}
     void begin(uint16_t port=0);
+    void begin(uint16_t port, int reuse_enable);
     void setNoDelay(bool nodelay);
     bool getNoDelay();
     bool hasClient();