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/net.cc

@@ -1384,8 +1384,7 @@ cygwin_getpeername (int fd, struct sockaddr *name, socklen_t *len)
     }
   __except (EFAULT) {}
   __endtry
-  syscall_printf ("%R = getpeername(%d) %p", res, fd,
-  		  (fh ? fh->get_socket () : (SOCKET) -1));
+  syscall_printf ("%R = getpeername(%d)", res, fd);
   return res;
 }