Unlock IPv6 sysctl net.inet6.ip6.forwarding from net lock.

Unlock IPv6 sysctl net.inet6.ip6.forwarding from net lock.

Use atomic operations to read ip6_forwarding while processing packets
in the network stack.
To make clear where actually the router property is needed, use the
i_am_router variable based on ip6_forwarding.  It already existed
in nd6_nbr.  Move i_am_router setting up the call stack until all
users are independent.
The forwarding decisions in pf_test, pf_refragment6, ip6_input do
also not interfere.
Use a new array ipv6ctl_vars_unlocked to make transition of all the
integer sysctls easier.  Adapt IPv4 to the new style.

OK mvs@