fix previous commit in case of access to non existent backend (e.g. rootDSE); add (and document) authzID/DN support

pwdHistory, and pwdReset NO-USER-MODIFICATION

which should help in debugging schema development for back-config.
The check essentially verifies, when a duplicate attributeType is found,
that the new attributeType definition is an exact duplicate of the
definition already present in the schema.

and we're also a consumer.

fly if a modify is attempted and it does not exist.