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.