Do not bother with bogonsv6 if IPv6 allow is off

On small systems with the default 200000 table entries, I get "error loading the rules" when it tries to load bogonsv6 (which has 72004 entries as at 24 March 2013). If Allow IPv6 is turned off, a block all IPv6 rule is in place anyway, so the bogonsv6 table is unnecessary.
This change prevents the bogonsv6 table from being created if Allow IPv6 is off.
It still downloads the table and saves it to /etc/bogonsv6 (if it looks like it would fit with the defined state table entries) - that way if the user turns on Allow IPv6, the bogonsv6 table will be immediately effective.
Happy to have comments about this. I think this is a piece of the IPv6 "functionality" that can be safely disabled when Allow IPv6 is off. It will help people with low-memory systems who have no IPv6 yet.