Show managed links in "brew cask list"

Fixes #2261 (note: user raises two issues)

`brew cask list` output can be confusing, because raw cask contents can contain symlinks.  @AlJohri (very reasonably) interpreted those symlinks as a confirmation of the (quite different) symlinks managed by brew-cask.

- proposed fix is to return multiple labeled sections for `brew cask list`.
- requires diverging from the semantics of `list` in Homebrew (which only shows "raw contents" with no labeling)
- could be improved by analysis of managed links which are expected-but-absent
- original report was exacerbated by file named <space_character> in the Cask, which is not addressed here
- it's not a minor bug: seen in popular casks `google-chrome` and `evernote`