Feature: allow the use of STUN to connect client and server together

This method doesn't require port-forwarding to be used, and works for
most common NAT routers in home setups. But, for sure it doesn't work
for all setups, and not everyone will be able to use this.

src/network/core/os_abstraction.h

@@ -196,6 +196,7 @@ static inline socklen_t FixAddrLenForEmscripten(struct sockaddr_storage &address
 
 bool SetNonBlocking(SOCKET d);
 bool SetNoDelay(SOCKET d);
+bool SetReusePort(SOCKET d);
 NetworkError GetSocketError(SOCKET d);
 
 /* Make sure these structures have the size we expect them to be */