Ensure our network layer is properly cleaned up before PuTTY exits.

Specifically, we explicitly closesocket() all open sockets, which
appears to be necessary since otherwise Windows sends RST rather
than FIN. I'm _sure_ that's a Windows bug, but there we go.

[originally from svn r1574]

network.h

@@ -62,6 +62,7 @@ struct plug_function_table {
 
 
 void sk_init(void);		       /* called once at program startup */
+void sk_cleanup(void);		       /* called just before program exit */
 
 SockAddr sk_namelookup(char *host, char **canonicalname);
 void sk_getaddr(SockAddr addr, char *buf, int buflen);