IPv6 firewall rules can be incomplete when using PPP

start_firewall is called by both rc and ipv6-up. If these two calls overlap, it could cause the filter_rules_ipv6 to be incomplete, which causes loss of IPv6 connectivity (although ping works)
Merge pull request #705 from pinwing/ipv6

Sometimes IPv6 firewall rules are incomplete when using PPPoE (timing issue)