Minimize inclusion of bogonsv6

If "Allow IPv6" is on, but actually there is no enabled interface with "Block bogon networks" enabled, then we also do not need to include the bogonsv6 table into pf.
This allows some more flexibility for users to leave "Allow IPv6" checked, but still not use up memory for bogonsv6.
This is a filter.inc integrated with Main. Can it also be back-ported/merged to 2.1 branch?
(There are now some small diffs of filter.inc between Main and 2.1, but I guess GitHub can easily sort that automatically?)