[thomasjm]

Adds more checks to nix flake check and fixes what they found:

• Docs build with warnings as errors (fixed 16 broken links).
• rust_2018_idioms, unused_qualifications and unsafe_op_in_unsafe_fn.
• Clippy's cast lints: casts that could truncate or change sign now use checked conversions, or say why they're fine.
• Every unsafe block needs a // SAFETY: comment. Two were removed as unnecessary, and FdTransport::from_raw_fd now owns its fd from the start (it used to leak it on errors).
• cargo-deny (licenses, banned crates, sources; deny.toml) and cargo-audit with a pinned advisory-db flake input. Updated tokio and rustix past unsound-code advisories; the fuser one doesn't apply since we build it without libfuse.