Do not bother with bogonsv6 if IPv6 allow is off

The rules already block all IPv6 when IPv6 allow is off, so it is a waste of 70,000+ table entries of V6 bogons. On small systems this matters, and I sometimes get errors loading rules with the default table entries limit even though I don't care for V6 stuff.
Do not bother with bogonsv6 if IPv6 allow is off

Only implement the bogonsv6 table if IPv6 allow is on. Otherwise, if it looks like there would be room for the table in pf, then save the file anyway to /etc/bogonsv6 so it is there if the user switches on IPv6 allow.
Improve bogonsv6 checks during update

Improvements to using pfctl to check the existence of the bogonsv6 table.
Merge pull request #497 from phildd/master

Do not bother with bogonsv6 if IPv6 allow is off