move compare to frontend (disabled by default, only invoked if backend doesn't...

move compare to frontend (disabled by default, only invoked if backend doesn't provide hook); honor ITS#3472 (disclose access check); fix ITS#3521 as well (check access to hasSubordinates); fix a problem in backend_attribute() with operational attrs; add backend_access() helper

servers/slapd/proto-slap.h

@@ -291,6 +291,15 @@ LDAP_SLAPD_F (int) backend_attribute LDAP_P((
 	slap_access_t access
 ));
 
+LDAP_SLAPD_F (int) backend_access LDAP_P((
+	Operation		*op,
+	Entry			*target,
+	struct berval		*edn,
+	AttributeDescription	*entry_at,
+	struct berval		*nval,
+	slap_access_t		access,
+	slap_mask_t		*mask ));
+
 LDAP_SLAPD_F (int) backend_operational LDAP_P((
 	Operation *op,
 	SlapReply *rs