Bug#268631: force preference of IPv4 over IPv6

[Juliusz Chroboczek <Juliusz.Chroboczek@pps.jussieu.fr>]

> v4 is much faster on this route than v6.  I suspect this is typical of
> IPv6-enabled machines, and will remain so for at least ten years.

Please do not do that in APT.  It doesn't belong in APT, it should be
a global, system-wide preference.  I don't feel like adjusting the
preferences of every single network program whenever I switch from
a good IPv6 to a bad IPv6 network.

Barak: if IPv6 is much slower than IPv4, please adjust the preferences
in /etc/gai.conf.  Just say

  precedence ::ffff:0:0/96  100

                                        Juliusz