Improve purge handling of unknown files

[zooba]

Currently, uninstall --purge will remove recognised installs and the known directories, but we should do a better job of cleaning up unrecognised files.

This is more complicated than just rmtree(...), because we want to ensure we don't purge any runtimes that are in use. So we still need to keep the atomic-unlink for those, and only apply special handling to other directories.

It might also be worth having a purge option that keeps recognised installs, so that users can clean up broken ones without affecting the good ones.