Convert vlan(4) to new ifnet KPI. The vlan(4) still includes if_var.h,
and knows struct ifnet, which is okay for an interface driver integrated
in stack.

The core idea is that if_vlantrunk structure now carries struct ifops,
which are composed of ifops from the parent interface and a couple of
vlan-specific ops. All vlan instances sitting on the same trunk share
the ops.

Another important change is that vlan(4) is more divorced with the
Ethernet layer, than it used to be. It doesn't use ether_ifattach().
However, running on Ethernet interfaces it will implicitly call
ether_input().

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