[Patch] Fix libstdc++/9533

[Paolo Carlini]

Nathan Myers wrote:

>Almost anything.  This is good, because now we don't need the 
>block() and no_block() functions; it all goes in showmanyc().
>showmanyc() is supposed to tell all it can find out about how
>many bytes are really there, not just how many it has buffered
>or will buffer, but it shouldn't take too long about it.
>
... on the other hand, a _very_ nice thing of this approach is
that in one shot we could fix also the long standing QoI issue
libstdc++/7744 (in_avail always returning zero for cin).

Paolo.