add fopencookie

See https://issues.dlang.org/show_bug.cgi?id=13755
I tried. It doesn't work because extern C functions aren't closures.

stdio.d:45: error: function stdio.__unittestL31_1.specialClose cannot access frame of function stdio.__unittestL31_1

I wanted to keep the bindings minimal. I think if fancy wrappers allowing closures etc are required they can go in a different module (for example in phobos). So the unittest has to look like that or not test anything useful (it already tests very few things).

What do you think?