[Patch] Fix libstdc++/9533

[Paolo Carlini]

Nathan Myers wrote:

>Ideally showmanyc() should try to do an fstat64() on the file 
>descriptor and report the result, if any.
>
...

>It also ought to try 
>
>  ::recv(this->fd(), 0, 0, (MSG_DONTWAIT|MSG_PEEK|MSG_NOSIGNAL))
>
>in case it's a socket or pipe.
>
Another observation which comes to my mind: that this approach is
really low level wrt the underflow (hack, indeed) can be seen clearly
from the fact that in this way we get the size in bytes, not the
number of chars in the internal representation.
Therefore, it seems to me, showmanyc will have to use also the
codecvt information.

Paolo.