[patch] Split openFileForWrite into windows and unix versions

[Rafael Espíndola]

This is hopefully the last file handling patch for now. It is similar
to 186511, but for creating files for
writing.

It is interesting that all tests pass even when I don't add _O_TEXT to
_open_osfhandle. Are we missing tests or we never depend on windows
handling a file as text?

Cheers,
Rafael
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