UDP proxying should track and decrement TTL

When UDP proxying from A-B it's really unfortunate when bad or misconfigured hardware ends up routing your packets in a loop. Decrementing TTL means you don't end up with that loop being infinite and destroying your CPU.

useful kernel options:

setsockopt(fd, IPPROTO_IP, IP_RECVTTL, &get_ttl, sizeof(get_ttl));

setsockopt(fd, IPPROTO_IPV6, IPV6_RECVHOPLIMIT, &get_ttl,
sizeof(get_ttl));

kMinCmsgSpaceForRead += CMSG_SPACE(sizeof(int)) // TTL

if ((cmsg->cmsg_level == IPPROTO_IP && cmsg->cmsg_type == IP_TTL) ||
(cmsg->cmsg_level == IPPROTO_IPV6 && cmsg->cmsg_type == IPV6_HOPLIMIT)) {
ttl = reinterpret_cast(CMSG_DATA(cmsg))));
}