Change replace ACL semantics from U-Mich historical behavior.

U-Mich allows someone with selfwrite to use replace all values
of an attribute with a value containing their DN.  Which, of course,
could than be deleted.  This behavior was carried forward in all
versions of OpenLDAP.

The new semantics separate checks for deleting all existing values
and adding new values.  It is more logical and more inline with
the common use of selfwrite.