redox: Require scheme for path to be absolute

redox: Require scheme for path to be absolute

Redox paths are problematic. It would make sense to add a `Scheme`
variant to the `std::path::Component` enum; but that would presumably be
a breaking change due to exhaustive matching. Alternately it could use
the existing `Prefix` variant, like Windows, but none of the existing
types of prefix make sense, Redox only has one kind, and adding a new
variant to that enum has the same issue as `Component`.