Fix up DISPLAYTITLE and enable per default:

Fix up DISPLAYTITLE and enable per default:
* Clean up the mess in ParserOutput
* Reject (ignore) custom titles which don't normalise to the same as the current page -- THIS IS IMPORTANT OTHERWISE LINKING GOES TO POT (and not the good kind of pot)
[WARNING: Touches parser version. Old caches will be expired. You might wish to undo this and add some temporarily backwards-compatibility for a few days.]