Safety violation related to the communication file descriptors

The way we [instantiate][1] `std::fs::File` objects from file descriptors passed by Fleetspeak is a violation of safety invariants as we cannot know whether it is truly valid. We should somehow verify that the descriptor is indeed valid.

[1]: https://github.com/google/fleetspeak-rs/blob/d031cfc8ee07aa0670e0040d24ab99dd689e4124/crates/fleetspeak/src/lib.rs#L270-L281