gaol does not set sandboxed process current working directory

[rocallahan]

After spawning a jailed subprocess, its current working directory is unchanged, so it can actually be outside the subprocess' sandbox which I believe means it can access files outside the sandbox. This is bad.

[rocallahan]

platform/linux/namespace.rs does

 match env::set_current_dir(&Path::new(".")) {

That's a no-op AFAIK. It should probably chdir to root.