[Patch] Fix libstdc++/11378 (take2) + xsputn optmizations

[Pétur Runólfsson]

Nathan Myers wrote:
> That this is an optimization is not expected to be obvious.

Indeed it isn't obvious :-)

> As Paolo noted, it remains sub-optimal until after we get filebuf using 
> writev(), so that flushing existing characters may be combined with 
> writing out the new characters.

Thanks for the explanation. 

> Of course all these optimizations can be done without breaking the ABI,
> so could be done at any time.  Do we all agree that large-file support 
> is much more urgent, by that criterion?

Absolutely.

Petur