merge unstow_orig.t into unstow.t and fix unstowing logic

There was a ton of duplication which is not maintainable, so refactor
everything into a single test which still covers the differences.

This in turn revealed some issues in the unstowing logic:

- We shouldn't conflict if we find a file which isn't a link or a
  directory; we can just skip over it.

- Unstowing with `--dotfiles` was using the wrong variable to obtain
  the package path, and as a result having to perform an unnecessary
  call to `adjust_dotfile()`.

So fix those at the same time.

MANIFEST

@@ -50,7 +50,6 @@ t/stow.t
 t/rc_options.t
 t/testutil.pm
 t/unstow.t
-t/unstow_orig.t
 tools/get-version
 THANKS
 TODO