Add vlan support and hardware offload capabilities to agr.
These changes allow vlans to be layered above agr, with the attach
and detach propogated to the member ports in the aggregation.
Note the agr interface must be up before the vlan is attached.

Adds SIOCINITIFADDR support to the wm driver for setting the AF_LINK
address, necessary for agr to be able to set the mac addresses of each
port to the agr address (i.e. so it can receive all intended traffic
at the hardware level).

Adds support for disabling the LACP protocol by setting LINK1 on the agr
interface (e.g. ifconfig agr0 link1).

In consultation with tls@.

(darran)