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

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().