nvm_ls collects installed versions with `find <dirs>/* -name . -o -type d -prune -o -path ...`,
and `-type d` is false for a symlink,
so a version dir that links to another volume was skipped from the listing entirely
- while `nvm version`, `nvm which` and `nvm use` all kept working on it.
Have that find follow symlinks,
which makes linked-in versions visible and still ignores a broken link.
Fixes#617.