IPC Message::push_move_handle might cause handle leak

[todo]

The push_move_handle function immediately downcasts the handle to a mere int, and forgets the (droppable) handle. This might cause a leak if the underlying IPC message is not sent. It'd be better to keep the handle around as long as possible. In fact, closing the handle after it's been moved might not be such a bad idea. After all, handles are guaranteed not to get reused.

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https://github.com/roblabla42/KFS/blob/adee47274ba5c0dc0ef112ce8933becd63d3da77/libuser/src/ipc/mod.rs#L362-L372

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This issue was generated by todo based on a TODO comment in adee472 when #90 was merged. cc @roblabla.