Use lladdr_event to propagate gratiotus arp.

[melifaro]

Summary

Currently we explicitly call ARP code from if_setlladdr() inside if.c.
We already have lladdr_event for internal subscribers like vlan.
Given that, add remaining hooks to make event work reliably (depends on D4004) and
convert ARP handler to use event instead of direct calls inside if.c

Test Plan

Create vlan on top of physical interface, run tcpdump for arp ethertype on both physical and vlan interfaces.
Change physical interface lladdr to different value, wait a bit and change it back.
You should see 2 arp who-has on both physical and vlan interfaces.

More complicated lagg-related examples exists in D4004.

[ae]

Looks reasonable for me.

[melifaro]

Pre-commit sync. Split announce logic from event handler. Register handler
only for default VNET.