Do a database update to fix random questions with the wrong parent.

In Moodle, all random questions should have question.parent set to be the same as question.id. One effect of MDL-5482 is that this will not be true for questions that were backed up then restored. The probably does not cause many problems, except occasionally, if the bogus question.parent happens to point to a multianswer question type, or when you try to do a subsequent backup. Anyway, these question.parent values should be fixed.

Unfortunately, at the moment, the main version number is 2007101509, so there is not a spare number before 1.9.1. However 1.9.1 is meant to be close, so I may check in the update code with an if ($oldversion < 2007101510) but not upgrade version.php. That will mean that this upgrade gets run the next time anyone else updates the version number, which is good enough on the branch. I will update the version number on HEAD.