Cygwin: encapsulate Winsock based fhandler_socket classes

Insert another class fhandler_socket_wsock between fhandler_socket
and fhandler_socket_inet/fhandler_socket_local.

Also, add a new method fhandler::is_wsock_socket to allow asking
for sockets in general (is_socket) vs. Winsock-based sockets
(is_wsock_socket).

This allows to develop a new handler_socket_unix class as derived
class from fhandler_socket without any trace of wsock code left
in fhandler_socket.

While this is basically a temporary measure at this time, it may
prove useful for later interoperability with the upcoming Windows 10
AF_UNIX implementation at one point.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>

winsup/cygwin/fhandler.cc

@@ -1567,7 +1567,7 @@ fhandler_base::fork_fixup (HANDLE parent, HANDLE &h, const char *name)
 {
   HANDLE oh = h;
   bool res = false;
-  if (/* !is_socket () && */ !close_on_exec ())
+  if (!close_on_exec ())
     debug_printf ("handle %p already opened", h);
   else if (!DuplicateHandle (parent, h, GetCurrentProcess (), &h,
 			     0, !close_on_exec (), DUPLICATE_SAME_ACCESS))