- Can up/down a link.
- Can set MAC on a link, if it's a phy.
- Can set MTU on a link.
- Can delete link (including phy).
Because link state and mtu changes tend to go around in circles (from
netlink -> vpp; and then with lcp-sync on, as well from vpp -> netlink)
when we consume a batch of netlink messages, we'll temporarily turn off
lcp-sync if it's enabled.
TODO (in the next commit), the whole nine yards of creating interfaces
in VPP based on NEWLINK vlans that come in. Conceptualy not too
difficult: if NEWLINK doesn't have a LIP associated with it, but it's a
VLAN, and the parent of the VLAN is a link which _does_ have a LIP, then
we can create the subint in VPP in the correct way.
