Move `fd` into `std::sys`
Move platform definitions of `fd` into `std::sys`, as part of https://github.com/rust-lang/rust/issues/117276.
Unlike other modules directly under `std::sys`, this is only available on some platforms and I have not provided a fallback abstraction for unsupported platforms. That is similar to how `std::os::fd` is gated to only supported platforms.
Also, fix the `unsafe_op_in_unsafe_fn` lint, which was allowed for the Unix fd impl. Since macro expansions from `std::sys::pal::unix::weak` trigger this lint, fix it there too.
cc `@joboet,` `@ChrisDenton`
try-job: x86_64-gnu-aux
a64ccf4a46