Fix IPv6 sockets on Windows not being dual-stack

On Linux, all IPv6 sockets are dual-stack per default; on Windows, they
are not. It's still a good idea to set the option on Linux as well as
the default value can be changed.

src/network/C4NetIO.cpp

@@ -674,6 +674,17 @@ C4NetIO::~C4NetIO()
 
 }
 
+bool C4NetIO::EnableDualStack(SOCKET socket)
+{
+	int opt = 0;
+	if (setsockopt(socket, IPPROTO_IPV6, IPV6_V6ONLY, reinterpret_cast<char*>(&opt), sizeof(opt)) == SOCKET_ERROR)
+	{
+		SetError("could not enable dual-stack socket", true);
+		return false;
+	}
+	return true;
+}
+
 void C4NetIO::SetError(const char *strnError, bool fSockErr)
 {
 	fSockErr &= HaveSocketError();
@@ -1097,6 +1108,9 @@ bool C4NetIOTCP::Connect(const C4NetIO::addr_t &addr) // (mt-safe)
 		return false;
 	}
 
+	if (!EnableDualStack(nsock))
+		return false;
+
 #ifdef STDSCHEDULER_USE_EVENTS
 	// set event
 	if (::WSAEventSelect(nsock, Event, FD_CONNECT) == SOCKET_ERROR)
@@ -1424,6 +1438,8 @@ bool C4NetIOTCP::Listen(uint16_t inListenPort)
 		SetError("socket creation failed", true);
 		return false;
 	}
+	if (!EnableDualStack(lsock))
+		return false;
 	// To be able to reuse the port after close
 #if !defined(_DEBUG) && !defined(_WIN32)
 	int reuseaddr = 1;
@@ -1796,6 +1812,9 @@ bool C4NetIOSimpleUDP::Init(uint16_t inPort)
 		return false;
 	}
 
+	if (!EnableDualStack(sock))
+		return false;
+
 	// set reuse socket option
 	if (::setsockopt(sock, SOL_SOCKET, SO_REUSEADDR, reinterpret_cast<char *>(&fAllowReUse), sizeof fAllowReUse) == SOCKET_ERROR)
 	{

src/network/C4NetIO.h

@@ -273,6 +273,9 @@ public:
 protected:
 //	virtual SOCKET CreateSocket() = 0;
 
+	// Makes IPv4 connections from an IPv6 socket work.
+	bool EnableDualStack(SOCKET socket);
+
 	// *** errors
 protected:
 	StdCopyStrBuf Error;