Multianswer questions get broken by backup and restore

When multianswer questions are backed up then restored, the question.parent field is not correctly recoded during the restore.

Fortunately, there is enough information in the mdl_question_multianswer.sequence field that we should be able to fix up any existing broken questions. See http://docs.moodle.org/en/Development:Question_bank_consistency_check for some thoughts.