[Closes #106] Hold `Arc<SpinLock<OpenedFileTable>>` instead of just `SpinLock<>`

[michalfita]

Description

Part of preparation to support the whole clone3() syscall. This change allows threads to keep access to the shared table of opened files (in #88 and #115), but for now it retains just cloning the table into new Arc<SpinLock<>> on fork().

Pre-Submission Checklist

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• [x] I assert this contribution was authored by me.
• [x] I license this contribution under the license of this project.
• [x] The PR title describes your changes briefly and uses the present tense, without a trailing full stop.
• [x] The PR description describes the reason why we need the change.
• [x] This is an isolated change. No multiple changes and no unrelated changes are included.
• [ ] Fixed all cargo clippy warnings.
• [ ] Applied rustfmt.

[nuta]

Merged. Thanks!