In many cases we want to treat IPv4 and IPv6 generically. That looks nicer
if we distingish by an @addr_family integer, instead of a boolean.

Replace the @is_ipv6 boolean with an @addr_family paramter. The @is_ipv6
boolean is inconsistent with other places where we use @is_ipv4 to
indicate the opposite. Eventually, we should use @addr_family
everywhere.

Also, at the call site it's not immediately clear what TRUE/FALSE means,
here AF_INET/AF_INET6 is better.