Define cork() as pure virtual in Socket class

This makes it possible to create a derived class from Socket which is
not TCP based, without having VNCSConnectionST.cxx trying to call
setsockopt() on a non-socket.

common/network/Socket.h

     // if shutdown() is overridden then the override MUST call on to here
     virtual void shutdown() {isShutdown_ = true;}
     bool isShutdown() const {return isShutdown_;}
+    virtual bool cork(bool enable) = 0;
 
     // information about this end of the socket
     virtual int getMyPort() = 0;

common/network/TcpSocket.cxx

   return true;
 }
 
-bool TcpSocket::cork(int sock, bool enable) {
+bool TcpSocket::cork(bool enable) {
 #ifndef TCP_CORK
   return false;
 #else
   int one = enable ? 1 : 0;
-  if (setsockopt(sock, IPPROTO_TCP, TCP_CORK, (char *)&one, sizeof(one)) < 0)
+  if (setsockopt(getFd(), IPPROTO_TCP, TCP_CORK, (char *)&one, sizeof(one)) < 0)
     return false;
   return true;
 #endif

common/network/TcpSocket.h

     virtual bool sameMachine();
 
     virtual void shutdown();
+    virtual bool cork(bool enable);
 
     static bool enableNagles(int sock, bool enable);
-    static bool cork(int sock, bool enable);
     static bool isListening(int sock);
     static int getSockPort(int sock);
   private:

common/rfb/VNCSConnectionST.cxx

 
     // Get the underlying TCP layer to build large packets if we send
     // multiple small responses.
-    network::TcpSocket::cork(sock->getFd(), true);
+    sock->cork(true);
 
     while (getInStream()->checkNoWait(1)) {
       if (pendingSyncFence) {
     }
 
     // Flush out everything in case we go idle after this.
-    network::TcpSocket::cork(sock->getFd(), false);
+    sock->cork(false);
 
     inProcessMessages = false;
 
   // mode, we will also have small fence messages around the update. We
   // need to aggregate these in order to not clog up TCP's congestion
   // window.
-  network::TcpSocket::cork(sock->getFd(), true);
+  sock->cork(true);
 
   // First take care of any updates that cannot contain framebuffer data
   // changes.
   // Then real data (if possible)
   writeDataUpdate();
 
-  network::TcpSocket::cork(sock->getFd(), false);
+  sock->cork(false);
 }
 
 void VNCSConnectionST::writeNoDataUpdate()