cmd/uninstall: note etc files that stay around.

We don't remove `etc` files on uninstall. Now we have `pkgetc`, though,
we have a pretty decent guess at what files in `etc` may belong to a
given package and can warn about them being left around on uninstall.

Thoughts: should we do the same thing for `var`? I don't see it being
used nearly as consistently.

Library/Homebrew/cmd/uninstall.rb

@@ -80,6 +80,27 @@ module Homebrew
               puts "#{keg.name} #{versions.to_sentence} #{"is".pluralize(versions.count)} still installed."
               puts "Run `brew uninstall --force #{keg.name}` to remove all versions."
             end
+
+            paths = f.pkgetc.find.map(&:to_s) if f.pkgetc.exist?
+            if paths.present?
+              puts
+              opoo <<~EOS
+                The following #{f.name} configuration files have not been removed!
+                If desired, remove them manually with `rm -rf`:
+                  #{paths.sort.uniq.join("\n  ")}
+              EOS
+            end
+
+            unversioned_name = f.name.gsub(/@.+$/, "")
+            maybe_paths = Dir.glob("#{f.etc}/*#{unversioned_name}*") - paths
+            if maybe_paths.present?
+              puts
+              opoo <<~EOS
+                The following may be #{f.name} configuration files and have not been removed!
+                If desired, remove them manually with `rm -rf`:
+                  #{maybe_paths.sort.uniq.join("\n  ")}
+              EOS
+            end
           end
         end
       end