[RFC] 9533: sixth iteration

[Pétur Runólfsson]

> Here is take 6.
> 
> Paolo.

This looks great, but if this is meant for 3.3 this:

+	  if (__cvt.encoding() >= 0)
+	    __ret += _M_file.showmanyc_helper(__sync) / __cvt.max_length();

should be changed to:

+	  if (__cvt.always_noconv())
+	    __ret += _M_file.showmanyc_helper(__sync);

since codecvt<wchar_t, char, mbstate_t>::encoding() and max_length() in
3.3 return garbage (they are not perfect in 3.4 either :-).

Petur