Define _AT_NULL_PATHNAME_ALLOWED

Define _AT_NULL_PATHNAME_ALLOWED

Cygwin needs an internal flag to allow specifying an empty pathname
in utimesat (GLIBC extension). We define it in _default_fcntl.h to
make sure we never introduce a value collision accidentally.
While at it, define the values as 16 bit hex values.

Reviewed-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>