Cause runtime probing of v6 addresses to fail, even if we can actually
make v6 sockets, but we do not have struct in6_pktinfo.

Add a check in configure.in for struct in6_pktinfo.

Use the result of that check to define (if needed) in6_pktinfo in net.h.

This change makes us build on platforms that have SOME ipv6 structures
(like in6_addr, etc) but not enough to be useful to us.

Note:  Before making an ipv6 socket the code should verify that ipv6
sockets can be made using isc_net_probeipv6().  It should also
check for v4 sockets using isc_net_probeipv4() to be consistant.