Add 'autoconf' and 'disable_ipv6' parameters to the IPv6 module.

The first controls if IPv6 addresses are autoconfigured from
prefixes received in Router Advertisements.  The IPv6 loopback
(::1) and link-local addresses are still configured.

The second controls if IPv6 addresses are desired at all.  No
IPv6 addresses will be added to any interfaces.
Signed-off-by: Brian Haley <brian.haley@hp.com>
Signed-off-by: David S. Miller <davem@davemloft.net>