udp_io, traceroute: Standardise IPv6 PKTINFO handling to be portable

udp_io, traceroute: Standardise IPv6 PKTINFO handling to be portable

The current standard (RFC 3542) is for IPV6_RECVPKTINFO to be given to
setsockopt, and IPV6_PKTINFO to be used as the packet type. Previously,
RFC 2292 required IPV6_PKTINFO to be used for both, but RFC 3542
re-purposed IPV6_PKTINFO when given to setsockopt. The special
Linux-specific IPV6_2292PKTINFO has the same semantics as IPV6_PKTINFO
in RFC 2292, but was introduced at the same time as IPV6_RECVPKTINFO.

Therefore, if we have IPV6_RECVPKTINFO available, we can use the RFC
3542 style, and if not, we assume that only the RFC 2292 API is
available, using IPV6_PKTINFO for both.

Signed-off-by: James Clarke <jrtc27@jrtc27.com>
Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>