When sending TCP Segments from the TCP SYN cache (like SYN-ACK segments) over IPv6, the traffic class in the IPv6 header was not initialised.

This patch ensures that the traffic class is 0, when sending TCP segments.

For the ECN bits, this is the correct value as specified in RFC 3168.

When a specific DSCP is requested by the application using

uint32_t tclass;
setsockopt(listen_fd, IPPROTO_IPV6, IPV6_TCLASS, &tclass, sizeof(tclass));

on the listening socket, this is not working correctly (and it wasn't).
It will be addressed in a separate review.