Set environment variables needed for bash, zsh completion in opam env

[WardBrian]

Would close #6427

This updates opam env to set XDG_DATA_DIRS (from the XDG base directory specification) to include the current switch's share root, and FPATH to include %{share}%/zsh/site-functions when the shell is zsh

The former works with bash-completion, which looks in the XDG_DATA_DIRS entries for bash-completion/completions sub-directories, while the later sets up zsh's built-in completion mechanisms to look in this location.

Questions for reviewers --

1. I currently only modify FPATH on zsh, but I think it would be safe to do it generically (the only other shell that seems to assign meaning to the FPATH variable is ksh, which opam doesn't support)
2. Assuming we want to keep FPATH zsh-specific, is it worth threading through a shell variable all the way from the CLI to where we compute these updates, or is it fine to keep the guess_shell_compat call?

[kit-ty-kate]

cc @hannesm @reynir do you have any thoughts about the security considerations of this feature by any chance?

[hannesm]

@kit-ty-kate thanks for asking, no I don't have an idea about this environment variable (and whether to prepend or append to it).

[reynir]

I don't know much about it. For bash-completion I suspect it could add completion scripts from installed opam packages, and I think those can run arbitrary commands. Then again I guess packages can already install arbitrary binaries shadowing common binaries such as ls. I'm not sure what the security model is or how that fits into that.

[WardBrian]

Even if you're sandboxing the build commands (so you can't be pwned by arbitrary badness hidden behind a make command), I still think it is currently the case that you can't install an opam package unless you trust it. It doesn't even have to shadow something like ls; presumably you're going to run the code from the package eventually, or else you wouldn't be installing it.

[dbuenzli]

Then again I guess packages can already install arbitrary binaries shadowing common binaries such as ls

Not to mention that arbitrary ocaml packages can actually set or extend theses variables themselves…