rework closesocket()

- make closesocket() the pure Win32 API call on Win32
- make closesocket() emulate the Win32 behaviour on all other OSes
extend and cleanup use of closesocket()

- add missing closesocket() calls for BindListenPort()
- always call closesocket() before printing any text (ensure consistent
  usage in the code)

misc small changes in net:
- remove unused CNode::Cleanup()
- add missing vhListenSocket.clear() to ~CNetCleanup()

changes in ConnectSocketDirectly() unrelated to closesocket():
- replace 3 following calls to WSAGetLastError() by storing result into
  nErr and use this in an if-clause
- convert an if() to an else if(), which removes an unneded check if the
  first case is true
add CSocket (RAII class that provides access to a SOCKET)