Fix MinGW build

Fix some issues with CAF on MinGW:
- Correctly initialize networking on Windows using WSAStartup()
- Rename `interface` member function to `message_types` because `interface` is
  a define on Windows (srsly Microsoft?)
- Cast to correct pointer type when using `send` or `recv`
- Fix some warnings caused by using `int` instead of `native_socket_t`

  The issues have been reported by GitHub user xmt.