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.

Author: ahorn42

libraries/WiFi/src/WiFiServer.cpp

@@ -63,6 +63,10 @@ WiFiClient WiFiServer::available(){
 }
 
 void WiFiServer::begin(uint16_t port){
+    begin(port, 1);
+}
+
+void WiFiServer::begin(uint16_t port, int enable){
   if(_listening)
     return;
   if(port){
@@ -72,6 +76,7 @@ void WiFiServer::begin(uint16_t port){
   sockfd = socket(AF_INET , SOCK_STREAM, 0);
   if (sockfd < 0)
     return;
+  setsockopt(sockfd, SOL_SOCKET, SO_REUSEADDR, &enable, sizeof(int));
   server.sin_family = AF_INET;
   server.sin_addr.s_addr = INADDR_ANY;
   server.sin_port = htons(_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();