Replace remaining process::exit with libc::_exit
Replace remaining process::exit with libc::_exit

Note that ptrace isn't documented as signal-safe, but it's supposed to
just be a light syscall wrapper, so it should be fine.

xd009642/nix,Susurrus/nix,berkowski/nix,Susurrus/nix,geofft/nix-rust,asomers/nix,xd009642/nix,Susurrus/nix,berkowski/nix,MarkusJais/nix-rust,carllerche/nix-rust,nix-rust/nix,asomers/nix,MarkusJais/nix-rust,geofft/nix-rust,MarkusJais/nix-rust,berkowski/nix,carllerche/nix-rust,geofft/nix-rust,carllerche/nix-rust,nix-rust/nix,xd009642/nix