[krbdev.mit.edu #2034] update ipv6 support to rfc3493

[Ken Raeburn via RT]

One change worthy of note is that AI_ADDRCONFIG is now in the list of
flags accepted by getaddrinfo (instead of just for getipnodeby*), and if
it doesn't cause problems with older implementations, we should use it.
 Not much point in looking up IPv6 addresses for KDCs if we only have
IPv4 available to reach them with.  (But: Do we get the right kind of
error if those KDCs only have IPv6 addresses?)