Dfc optimization

[Ben Wing]

>     Aidan> If mswindows-multibyte may involve the EBCDIC code pages,
>     Aidan> then it's probably not
>     Aidan> true. 
> http://www.unicode.org/Public/MAPPINGS/VENDORS/MICSFT/EBCDIC/
>     Aidan> for a list of those.
> 
> Doesn't matter, unless mswindows-multibyte starts in one of 
> them, or there's a way to switch out of ASCII using only the 
> printing characters.  The precondition is that the stream 
> contains _only_ printing ASCII (I assume that actually the 
> usual whitespace characters are allowed).

Aidan is actually right; you can create an `mswindows-multibyte' coding
system with any coding page, including EBCDIC ones, and then ASCII
conversion would not be the identity.