Useful for plugins that may wish to collude with the SASL library to,
for example, retrieve a user's authorization data from a Kerberos ticket
for access control purposes
preoperation plugin has removed all modifications and
thus there are none to pass to the backend.
Accept SSF as a valid parameter block value.
Fix completely broken slapi_access_allowed()
behaviour:
1. Plugins never return LDAP result codes, instead they return a
small integer (0 or -1, others for special cases)
2. Preoperation plugins can abort processing by returning a non-
zero value to the frontend
3. Postoperation plugins never abort processing (all are called)
and their return values are ignored
