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/host.c

@@ -40,14 +40,12 @@
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>
-#ifndef _WIN32
 #include <sys/types.h>
 #include <sys/socket.h>
 #include <netinet/in.h>
 #include <arpa/inet.h>
 #include <netdb.h>
 #include <unistd.h>
-#endif
 #include "misc.h"
 
 /* Portability cruft, should become unnecessary eventually */