vfscore: Support closing STD{IN,OUT,ERR} file descriptors about unikraft HOT 3 OPEN

I guess we would need to introduce a destructor table where libraries can register clean-up work like this. The dtors would get called on machine shutdown.

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@skuenzer This has nothing to do with cleaning up on shutdown. It just fixes calls such as close(STDIN_FILENO) when you just want to reuse the file descriptor.

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In such a case, I expect that we get a bit more clean solution with introducing fdtable: #52 . STDIN, STDOUT, STDERR should register a close fd-handler.

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