Fixed the way symmetrical many-to-many relations are recorded in the Options class.

These types of relations don't have reverse accessor names, so that name can be
used by a normal field on the model. Fixed #7107.


git-svn-id: http://code.djangoproject.com/svn/django/trunk@7764 bcc190cf-cafb-0310-a4f2-bffc1f526a37