> If, for example, the kernel doesn't error out on things it doesn't recognize (which does happen in some interfaces), then uv will mistakenly thing it has opened the fd O_CLOEXEC when it hasn't.

That would make it a kernel bug.  Returning EINVAL for unrecognized flags is mandated by POSIX.  Doesn't mean it's a bad thing to check, of course, but I would be taken aback if one of our supported platforms manages to screw up such a basic thing.