Root: support some stat-like APIs?

[cyphar]

At the moment we don't have APIs for "getters" like statx or accessat. While stuff like chmod(2) can be done with fchmodat, it is a bit less ergonomic to require someone to resolve the path themselves.

Then again, the overhead of doing a lookup of a parent is almost identical to doing a normal lookup so this is purely an ergonomics thing (though accessat would be a little annoying to implement for pre-5.8 kernels when it didn't accept AT_EMPTY_PATH).