Fix the mode being used in the implementation of open in the Fcntl portability header
Fix the mode being used in the implementation of open in the Fcntl portability header

Summary: The mode parameter to `open` under MSVC is not the same as the mode parameter everywhere else, so we need to do a bit of translation.

Reviewed By: yfeldblum

Differential Revision: D3651218

fbshipit-source-id: 80df1e15f34b8d66533256107d8c9218f757fde2

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