[Patch] Fix first part of libstdc++/13731

[Paolo Carlini]

Hi everyone, hi Loren,

this is the first part, dealing with write (the second part will affect
writev, and therefore xsputn).

I briefly recall that this work is needed because, when pipes are involved,
and n > PIPE_BUF, partial but succesful writes are possible: therefore we
must iterate 'til completion or error.

I would like you all to stress test in mainline this first part for a few
days, in the meanwhile work on the second part, which is less 
straighforward,
but still manageable since our writev are always of 1 or 2 buffers, not
fully general.

Tested x86-linux.

Paolo.

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