Doctor check for unlinked not-keg-only brews

Since these cause trouble.

Library/Homebrew/cmd/doctor.rb

@@ -842,6 +842,25 @@ def check_for_outdated_homebrew
   end
 end
 
+def check_for_unlinked_but_not_keg_only
+  unlinked = HOMEBREW_CELLAR.children.reject do |rack|
+    if not (HOMEBREW_REPOSITORY/"Library/LinkedKegs"/rack.basename).directory?
+      Formula.factory(rack.basename).keg_only? rescue nil
+    else
+      true
+    end
+  end.map{ |pn| pn.basename }
+
+  if not unlinked.empty? then <<-EOS.undent
+    You have unlinked kegs in your Cellar
+    Leaving kegs unlinked can lead to build-trouble and cause brews that depend on
+    those kegs to fail to run properly once built.
+
+        #{unlinked * "\n        "}
+    EOS
+  end
+end
+
 end # end class Checks
 
 module Homebrew extend self