mkdir_at on Unix is complicated

[Robert Collins]

See the code but in short we're working around race conditions such as privileged access target control via symlinks; while there aren't any TOCTOU's present, there is the possibility for creating a directory in an arbitrary path (which mkdir itself has and there is resistance to fixing in the Linux kernel community).

At this point there doesn't seem to be a better path, but should one arise we should use it.