Implement kill system call

[bayedieng]

This pr implements the kill syscall in order to further linux compatibility, omitting permission checks as Linux capabilities need to be implemented to do so.

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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.
• [x] Fixed all cargo clippy warnings.
• [x] Applied rustfmt.

[nuta]

@bayedieng I can't see any fixes for my comments. Perhaps did you forget to push commits?

[bayedieng]

@nuta updated.

[nuta]

Thanks @bayedieng!