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

[Paolo Carlini]

Pétur Runólfsson wrote:

>It seems to me that __limit should always be set to _M_buf_size - 1,
>that way the number of write calls is minimized (IIRC this is what
>libio does).
>
Well, if we all agree, it's easy to do this change, in fact the final 
code is *much* simpler.

However, I'd like to wait for Nathan's opinion. Some time ago he 
suggested a general scheme like that I have implemented above (to be 
fine tuned, of course).

Paolo.