Consider exposing openat/cwd abstraction from FileSystem

Need to think more deeply about which of them makes more sense from an API perspective: we can either expose a current-working-directory (cwd) abstraction and/or an openat style abstraction. Linux provides both, but imho that seems like overkill _at the litebox layer_ (instead, the shim should be in charge of ensuring translation).

I need to think about it a bit more, but I think the most general form is to have an `openat` and _not_ support a `cwd` at all _at the litebox layer (forcing usage of absolute paths always), but more thought is needed before finalizing this decision and implementing it.

Start by reviewing the FileSystem API and the issue's comparison of openat and current-working-directory abstractions. Resolve which abstraction belongs at the litebox layer versus the shim, then document the chosen API and implement it once the design is finalized.

Written by the indexing model from the issue text.