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.cpp

@@ -159,6 +159,23 @@ bool SetNoDelay(SOCKET d)
 #endif
 }
 
+/**
+ * Try to set the socket to reuse ports.
+ * @param d The socket to reuse ports on.
+ * @return True if disabling the delaying succeeded, otherwise false.
+ */
+bool SetReusePort(SOCKET d)
+{
+#ifdef _WIN32
+	/* Windows has no SO_REUSEPORT, but for our usecases SO_REUSEADDR does the same job. */
+	int reuse_port = 1;
+	return setsockopt(d, SOL_SOCKET, SO_REUSEADDR, (const char *)&reuse_port, sizeof(reuse_port)) == 0;
+#else
+	int reuse_port = 1;
+	return setsockopt(d, SOL_SOCKET, SO_REUSEPORT, &reuse_port, sizeof(reuse_port)) == 0;
+#endif
+}
+
 /**
  * Get the error from a socket, if any.
  * @param d The socket to get the error from.