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.
Merge pull request #714 from phil-davis/master

Minimize inclusion of bogonsv6