Support schemas that determine membership by having fields on the
user entity, instead of listing users on a groups entity. E.g. the
following schema is now supported when it wasn't previously:
cn=eric,cn=user,dn=exapmle,dn=com
objectClass=myPerson
cn: eric
uid: eric
email: eric@example.com
memberOf: foo
memberOf: bar
cn=foo,cn=group,dn=exapmle,dn=com
objectClass=myGroup
cn: foo
cn=bar,cn=group,dn=exapmle,dn=com
objectClass=myGroup
cn: bar