Bug#636871: provide a command-line switch to prefer IPv6 or IPv4

[Tim Small <tim@seoss.co.uk>]

Package: apt
Followup-For: Bug #636871

There are already system-wide mechanisms to change the default stack
which is used (/etc/gai.conf), and apt has: 

-o Acquire::ForceIPv4

and

-o Acquire::ForceIPv6

(see "man apt.conf" and #611891).

What might be better (and would be a useful addition) would be to add
fast fallback ("Happy Eyeballs" RFC6555) support to apt, especially when
run in interactive mode.  I'm not familiar with the code, but I believe
that libcurl includes RFC6555 support, and since apt uses libcurl, it
might be easy to turn this on (or it might not).