[PSA] USE=ipv6 enabled by default now

[Mike Frysinger]

ToT now enables USE=ipv6 by default.  we were manually setting USE=ipv6 on a number of packages as we noticed "hey we want IPv6 in xxx", so this flips the settings for all the rest.  except curl because there's a known issue there.

if anyone notices weird network behavior, prob want to look at this first :).

this shouldn't have any real user facing change in behavior though as Chrome itself has always supported IPv6.

-mike