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.