Basic implementation of TCP over XDP for Knot server

including connection handling for kxdpgun in TCP mode.

The implementation is not perfect!! But the interface with other Knot parts is stable.

src/knot/nameserver/axfr.c

@@ -191,6 +191,9 @@ int axfr_process_query(knot_pkt_t *pkt, knotd_qdata_t *qdata)
 			            knot_strerror(ret));
 			return KNOT_STATE_FAIL;
 		}
+	} else if (qdata->params->xdp_msg != NULL) {
+		qdata->rcode = KNOT_RCODE_SERVFAIL;
+		return KNOT_STATE_FAIL;
 	}
 
 	/* Reserve space for TSIG. */