Conform SLAPI to Netscape, iPlanet and Sun ONE Directory Server

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