pkstream/pkstream.c: Be more careful about handling address families.
* Introduce a concept of `known' address families. Currently, only
`AF_INET' is known.
* Filter `struct addrinfo' chains for known address families. If we
come up short, complain.
* Tweak `aihints' to arrange that addresses which are supposed to
match up actually will do: so server peer and bind addresses should
match; client bind and connect addresses should match; and UDP local
and remote addresses should match.
* Initialize address structures using the `ai_family' slot from the
appropriate `struct addrinfo' structure.