Assert on unknown isc_quota_attach() return values

The only values that the isc_quota_attach() function (called from
check_recursionquota() via recursionquotatype_attach_soft()) can
currently return are: ISC_R_SUCCESS, ISC_R_SOFTQUOTA, and ISC_R_QUOTA.
Instead of just propagating any other (unexpected) error up the call
stack, assert immediately, so that if the isc_quota_* API gets updated
in the future to return values currently matching the "default"
statement, check_recursionquota() can be promptly updated to handle such
new return values as desired.

lib/ns/query.c

@@ -6357,9 +6357,9 @@ check_recursionquota(ns_client_t *client, ns_query_rectype_t recursion_type) {
 				   "no more recursive clients (%u/%u/%u)",
 				   &client->manager->sctx->recursionquota);
 		ns_client_killoldestquery(client);
-		FALLTHROUGH;
-	default:
 		return (result);
+	default:
+		UNREACHABLE();
 	}
 
 	dns_message_clonebuffer(client->message);