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See http://lists.gnu.org/archive/html/emacs-devel/2013-02/msg00293.html
and the following discussion for the details.

 src/w32.c (set_errno): Reset h_errno and don't set it to any other
 value.  Set errno instead.
 (check_errno): Reset h_errno.
 (sys_socket, socket_to_fd, sys_bind, sys_connect)
 (sys_gethostname, sys_getservbyname, sys_getpeername)
 (sys_shutdown, sys_setsockopt, sys_listen, sys_getsockname)
 (sys_accept, sys_recvfrom, sys_sendto, fcntl, sys_read): Don't set
 h_errno.
 (sys_gethostbyname): Set h_errno only errors detected.