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

[Pétur Runólfsson]

> +	  // Measurement would reveal the best choice.
> +	  const size_t __chunk = 1ul << 10;
> +	  size_t __bufavail = this->epptr() - this->pptr();
> +
> +	  // Don't mistake 'uncommitted' mode buffered with unbuffered.
> +	  if (!_M_writing && this->_M_buf_size > 1)
> +	    __bufavail = this->_M_buf_size - 1;
> +
> +	  const size_t __limit = std::min(__chunk, __bufavail);
> +	  if (__n >= __limit)

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).

Petur