Improve error reporting as part of solving bug #13546 on MS-Windows.

Improve error reporting as part of solving bug #13546 on MS-Windows.

 src/w32.c (sys_socket, sys_bind, sys_connect, sys_gethostname)
 (sys_gethostbyname, sys_getservbyname, sys_getpeername)
 (sys_shutdown, sys_setsockopt, sys_listen, sys_getsockname)
 (sys_accept, sys_recvfrom, sys_sendto, fcntl): In case of failure,
 make sure errno is set to an appropriate value.  (Bug#13546)
 (socket_to_fd): Add assertion against indexing fd_info[] with a
 value that is out of bounds.
 (sys_accept): If fd is negative, do not set up the child_process
 structure for reading.