pkstream/pkstream.c: Introduce helper functions to fiddle with addresses.

The remaining places where pieces of addresses are fiddled with
directly, outside of these new functions, are:

  * in `doaccept', where we continue inspect the peer address to see if
    it's a wildcard, because we'll handle this in a very different way
    later; and

  * in `parseaddr', which needs to fill in addresses and port numbers.