Initial support for Windows

Initial support for Windows

In order to support Windows without adding a lot of #ifdefs all over
the code I decided to move all the inclusions of platform-specific
network headers into a separate include file. Windows use separate
subsystem for sockets (WinSock), and it is almost compatible with how
it works on Unix except from:

* The socket handle is a SOCKET and not a typical filedescriptor.
* It should be closed with closesocket() and not close().
* It does not set the global errno variable, but one should call
  WSAGetLastError() instead.