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

[Benjamin Kosnik]

> > I can live with names c_filebuf, FILE_filebuf, c_file_buf, or 
> > stdio_filebuf for that version.  I prefer the last as clearest.
> 
> Also, an historical note: the Jerry Schwarz version was called
> stdiobuf. I like that, too.

Yep. 

These interfaces can still be refined, but I thought it was a good idea
to get the ABI breaking bits in. 

-benjamin