NethServer::Directory (enforceAccessDirective): $field argument is now a full WHAT clause. Refs #2757

The old $field argument is now interpreted as a full WHAT clause. Only
the presence of an equal sign (=) triggers the old behaviour: "attrs="
is prepended.

Refer to slapd.access documentation for WHAT clause syntax.
Fix previous commit if WHAT is "*". Refs #2757