MEMBEROF: Keep inherited ghost users around on modify operation
It is possible to simply reset the list of ghost users to a different one
during a modify operation. It is also actually how we update entries that
are expired in the SSSD cache.
In this case, we must be careful and retain the ghost users that are not
native to the group we are processing but are rather inherited from child
groups. The intention of the replace operation after all is to set the
list of direct members of that group, not direct and indirect.