Returns one of the following values:
0 if the proposed modifications to the entry comply with the schema or if schema checking is turned off. The function also returns 0 if the entry has additional attributes not allowed by the schema and has the object class extensibleObject.
1 if the modifications cause the objectclass attribute or other required attributes to be missing, if the modifications add any attributes not allowed by the schema, or if the modifications result in multiple values for a single-valued attribute.