sources: Silence C4013 warnings on Visual Studio

Hi,

From the commit message:

The read(), close(), open(), lseek() functions are found in io.h on Visual Studio, which does not ship unistd.h, so include io.h on Windows if unistd.h is not found.

C4013 (aka implicit declaration of ...) warnings can often ring alarm bells.

Note that I only updated the sources where the C4013 warnings are raised unless io.h is included, not the ones that don't raise warnings without unistd.h included.

With blessings, thank you!