Created a shiny new abstraction for the socket handling. Has many

advantages:
 - protocol modules can call sk_write() without having to worry
   about writes blocking, because blocking writes are handled in the
   abstraction layer and retried later.
 - `Lost connection while sending' is a thing of the past.
 - <winsock.h> is no longer needed in most modules, because
   "putty.h" doesn't have to declare `SOCKET' variables any more,
   only the abstracted `Socket' type.
 - select()-equivalent between multiple sockets will now be handled
   sensibly, which opens the way for things like SSH port
   forwarding.

[originally from svn r744]

be_all.c

@@ -4,13 +4,6 @@
  */
 
 #include <windows.h>
-#ifndef AUTO_WINSOCK
-#ifdef WINSOCK_TWO
-#include <winsock2.h>
-#else
-#include <winsock.h>
-#endif
-#endif
 #include <stdio.h>
 #include "putty.h"