Walk with noxdev will visit (but not traverse) mountpoints

[jeckersb]

xref bootc-dev/bootc#2045 (comment)

I would say this is in an ambiguous state where I understand why we would visit the mountpoint, but it also mostly defeats the point of doing the walk with noxdev in the first place because it means that one would need to check every directory to see if it's a mount root before taking action on it.

Maybe not an outright bug, and the docs are currently correct in that they state that noxdev will prevent traversal across mount points, but at the very least maybe the docs should state explicitly that the mount point itself will be visited by the walk?

[cgwalters]

I think we should add .skip_mountpoints() with this new semantics. Should be relatively straightfoward.