Use src/lib/w32/w32sockets.c for client

Move client's w32_connect() to w32sockets.c.

Replace w32_recv() and w32_send() by read() and write().

Replace w32_close() by w32_close_function.

Replace call of WSAStartup() in w32_sysdep_init() by
w32_socket_init().

Remove the identical copies of fd_is_socket(),
w32_set_winsock_errno(), w32_socket().

src/client/login.c

@@ -38,8 +38,9 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#if !defined(_WIN32)
 #include <unistd.h>
+#ifdef _WIN32
+#include <windows.h>
 #endif
 #include "misc.h"
 #include "proto.h"