Replace readdir_r with readdir #2673
Description
Nix commit f9ebcb7 in 2018 added a dir module that internally made use of readdir_r. Even at that time readdir_r was already deprecated. But Nix accepted that change on the grounds that "the Rust standard library does it too". But that's no longer the case. The Rust standard library has switched to plain readdir for Fuchsia and Linux. It will probably soon switch for FreeBSD too. Nix should follow suit.