Mark `nix::unistd::daemon` unsafe and document why.

[blp]

Fixes: https://github.com/nix-rust/nix/issues/2663

What does this PR do

Marks nix::unstd::daemon unsafe and documents why.

Checklist:

• [x] I have read CONTRIBUTING.md
• [x] I have written necessary tests and rustdoc comments
• [x] A change log has been added if this PR modifies nix's API