Add reference counting to Defaults bindings.

Previously, we checked that the previous entry's binding pointer
was not the same while freeing.  However, to be able to merge
Defaults records we cannot rely on Defaults entries with the same
binding being immediately adjacent.  This removes the prev_binding
checks in favor of a reference count which allows us to plug the
memory leak in cvtsudoers when merging Defaults.

plugins/sudoers/parse_ldif.c

@@ -252,7 +252,8 @@ ldif_store_options(struct sudoers_parse_tree *parse_tree,
 	    sudo_fatalx(U_("%s: %s"), __func__,
 		U_("unable to allocate memory"));
 	}
-	TAILQ_INIT(d->binding);
+	TAILQ_INIT(&d->binding->members);
+	d->binding->refcnt = 1;
 	d->type = DEFAULTS;
 	d->op = sudo_ldap_parse_option(ls->str, &var, &val);
 	if ((d->var = strdup(var)) == NULL) {