convert interface address change hooks to tasks and a task_list.

this follows what's been done for detach and link state hooks, and
makes handling of hooks generally more robust.

address hooks are a bit different to detach/link state hooks in
that there's only a few things that register hooks (carp, pf, vxlan),
but a lot of places to run the hooks (lots of ipv4 and ipv6 address
configuration).

an address hook cookie was in struct pfi_kif, which is part of the
pf abi. rather than break pfctl -sI, this maintains the void * used
for the cookie and uses it to store a task, which is then used as
intended with the new api.

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