This allows using manual priorities for traps, which have a lower
base priority than the resulting IPsec policies. This could otherwise
be problematic if, for example, swanctl --install/uninstall is used while
an SA is established combined with e.g. IPComp, where the trap policy does
not look the same as the IPsec policy (which is now otherwise often the case
as the reqids stay the same).
It also orders policies by selector size if manual priorities are configured
and narrowing occurs.
The kernel policy now considers src and dst port masks as well as
restictions to a given network interface. The base priority is
100'000 for passthrough shunts, 200'000 for IPsec policies,
300'000 for IPsec policy traps and 400'000 for fallback drop shunts.
The values 1..30'000 can be used for manually set priorities.
This allows two CHILD_SAs with reversed subnets to install two FWD
policies each. Since the outbound policy won't have a reqid set we will
end up with the two inbound FWD policies installed in the kernel, with
the correct templates to allow decrypted traffic.
We can't set a template on the outbound FWD policy (or we'd have to make
it optional). Because if the traffic does not come from another (matching)
IPsec tunnel it would get dropped due to the template mismatch.
This allows us to install more than one FWD policy. We already do this
in the kernel-pfkey plugin (there the original reason was that not all
kernels support FWD policies).
