libutil: save cwd fd in restoreMountNamespace
libutil: save cwd fd in restoreMountNamespace

This doesn't work very well (maybe I'm misunderstanding the desired
implementation):

    : ~/w/vc/nix ; doas outputs/out/bin/nix --experimental-features 'nix-command flakes' develop -c pwd
    pwd: couldn't find directory entry in ‘../../../..’ with matching i-node

NixOS/nix,rycee/nix,NixOS/nix,rycee/nix,rycee/nix,rycee/nix,NixOS/nix,NixOS/nix,NixOS/nix,rycee/nix