subuser_load_all not returning the proper parent user ids and not returning userids if we have defined another relation

Issue Summary

Two problems in subuser_load_all:

1. If we set the children argument to FALSE to check get the parent user for a subuser we get the user id of the subuser itself. This is because we need to take the value from the other endpoint.
2. If we have more than one relation in the system where the enpoint user is allowed, for example a 'node -> belongs_to -> user', subuser_load_all would return also the node ids in the $user array.

I attach a patch right away, as I got an issue number.