[v3] remove non-standard basic_filebuf members, fd, FILE filebufs

[Benjamin Kosnik]

> I'm for anything which simplifies I/O debugging and documentation.

Me too. 

> The file/fd class merge you mentioned in a followup also sounds good.

c_filebuf, then? I'll add a ctor that sets file permissions as well.
I think those were the top three gripes then.

> Dare I suggest
> 
>     __glibcpp_class_requires(state_type, _SGIAssignableConcept)
> 
> say, just before the _M_state_cur assignment?

Ok, done.

I'll post a patch later.

-benjamin