Kerberos ticket policy can update policy in a user entry. This allowed
set/addattr to be used to modify attributes outside of the ticket policy
perview, also bypassing all validation/normalization. Likewise the
ticket policy was updatable by the user plugin bypassing all validation.
Add two new LDAPObject values to control this behavior:
limit_object_classes: only attributes in these are allowed
disallow_object_classes: attributes in these are disallowed
By default both of these lists are empty so are skipped.
ticket 744
