Move GenerateSelectSet logic to private method.

This separates the socket event collection logic from the logic
deciding which events we're interested in at all.

src/net.h

@@ -342,6 +342,7 @@ private:
     void DisconnectNodes();
     void NotifyNumConnectionsChanged();
     void InactivityCheck(CNode *pnode);
+    bool GenerateSelectSet(std::set<SOCKET> &recv_set, std::set<SOCKET> &send_set, std::set<SOCKET> &error_set);
     void SocketHandler();
     void ThreadSocketHandler();
     void ThreadDNSAddressSeed();