Add safe wrappers for getuid, geteuid, getgid, getegid
Add safe wrappers for getuid, geteuid, getgid, getegid

Fixes #213

brayniac/nix-rust,posborne/rust-nix,Susurrus/nix,nikklassen/nix-rust,xd009642/nix,kamalmarhubi/nix-rust,asomers/nix,MarkusJais/nix-rust,Susurrus/nix,xd009642/nix,carllerche/nix-rust,brayniac/nix-rust,kamalmarhubi/nix-rust,xd009642/nix,berkowski/nix,Susurrus/nix,asomers/nix,MarkusJais/nix-rust,geofft/nix-rust,posborne/rust-nix,geofft/nix-rust,MarkusJais/nix-rust,nix-rust/nix,kamalmarhubi/nix-rust,carllerche/nix-rust,nikklassen/nix-rust,brayniac/nix-rust,berkowski/nix,geofft/nix-rust,posborne/rust-nix,carllerche/nix-rust,nix-rust/nix,nikklassen/nix-rust,berkowski/nix