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

[Paolo Carlini]

Pétur Runólfsson wrote:

>>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.
>
Fine. Will be my top priority in the next few weeks.

By the way, I have changed my local copy of the fix for libstdc++/11378 
to use consistently the streamsize type, instead of size_t: seems better 
since both the return value and __n are streamsize.

Paolo.