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

[Ken Raeburn via RT]

RFC 2553 (Basic Socket Interface Extensions for IPv6) has been made
obsolete by RFC 3493.  Among other changes, two error codes have been
removed, EAI_ADDRFAMILY and EAI_NODATA; we use them.  We should check
for other relevant changes as well.

Ken