Explicitly disable IPV6_V6ONLY flag.

(It defaults to 1 for WinSock and 0 most other places.)

We try to do this no matter what, and we don't care if it fails; in case of
failure, it was either applied to a non-IPv6 socket where it would be
meaningless, or we're just going to have to live with what we got anyhow.

Reference Issue #84.
Reference Issue #58.

src/SDL_net.c

@@ -934,6 +934,9 @@ SDLNet_Server *SDLNet_CreateServer(SDLNet_Address *addr, Uint16 port)
         return NULL;
     }
 
+    int zero = 0;
+    setsockopt(server->handle, SOL_IPV6, IPV6_V6ONLY, &zero, sizeof (zero));  // if this fails, oh well.
+
     int rc = bind(server->handle, addrwithport->ai_addr, addrwithport->ai_addrlen);
     freeaddrinfo(addrwithport);
 
@@ -1284,6 +1287,9 @@ SDLNet_DatagramSocket *SDLNet_CreateDatagramSocket(SDLNet_Address *addr, Uint16
         return NULL;
     }
 
+    int zero = 0;
+    setsockopt(sock->handle, SOL_IPV6, IPV6_V6ONLY, &zero, sizeof (zero));  // if this fails, oh well.
+
     const int rc = bind(sock->handle, addrwithport->ai_addr, addrwithport->ai_addrlen);
     freeaddrinfo(addrwithport);