Fixed #14234 -- Re-validating a model instance added via ModelForm no longer throws spurious PK uniqueness errors. Thanks to David Reynolds and Jeremy Dunck.

Also moved Model._adding to Model._state.adding to reduce instance namespace footprint.

git-svn-id: http://code.djangoproject.com/svn/django/trunk@14612 bcc190cf-cafb-0310-a4f2-bffc1f526a37