Make --disable-ipv6 really do it

Make --disable-ipv6 really do it

Previously, it only made us stop *asking* the server for IPv6. If the server
gave us IPv6 addresses anyway on the basis that this is the 21st century and
there's no excuse for pretending otherwise (or, in practice, because my test
server is handing out hard-coded responses without looking at the request),
we were still actually using them.

Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>