Support negated filters on one-to-one and many-to-one Relationships

[camuthig]

This makes it possible to have a one-to-one and many-to-one Relationship with both multiple set to False and execute negated queries on it. This sort of relationship does not need to create a WHERE NOT EXIST subquery, and by setting the m2m variable the ORM will generate the correct query for us.

This does not solve the larger issue for a one-to-many or many-to-many relationship and negated queries.