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/libknot/xdp/tcp.c

@@ -459,8 +459,10 @@ int knot_xdp_tcp_timeout(knot_tcp_table_t *tcp_table, knot_xdp_socket_t *socket,
 			rem_node((node_t *)conn);
 			add_tail(&to_remove, (node_t *)conn);
 		} else if (now - conn->last_active >= close_timeout) {
-			rl.answer = XDP_TCP_CLOSE;
-			printf("close %hu timeout\n", be16toh(conn->ip_rem.sin6_port));
+			if (conn->state != XDP_TCP_CLOSING) {
+				rl.answer = XDP_TCP_CLOSE;
+				printf("close %hu timeout\n", be16toh(conn->ip_rem.sin6_port));
+			}
 		} else {
 			break;
 		}