home-environment: prevent delete of non-managed files
home-environment: prevent delete of non-managed files

When a file has disappeared between the previous and the next
generations then its symlink in `$HOME` is typically deleted. With
this commit we refuse to delete the path unless we are reasonably
certain it is a symlink into a Home Manager generation.

rycee/home-manager,rycee/home-manager,rycee/home-manager