Sync interface state from VPP to LCP

This is the first in a series of functions that aims to copy forward
interface changes in the VPP dataplane into the linux interfaces.

Capture link state changes (set interface state ...) and apply them
to Linux.

There's an important dissonance here:
- When Linux sets a parent interface up, all children also go up.

ip link set enp66s0f1 down
ip link add link enp66s0f1 name foo type vlan id 1234
ip link set foo down
ip link | grep enp66s0f1
9: enp66s0f1: <BROADCAST,MULTICAST> mtu 9000 qdisc mq state DOWN mode DEFAULT group default qlen 1000
61: foo@enp66s0f1: <BROADCAST,MULTICAST,M-DOWN> mtu 9000 qdisc noop state DOWN mode DEFAULT group default qlen 1000

ip link set enp66s0f1 up
ip link | grep s0f1
9: enp66s0f1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9000 qdisc mq state UP mode DEFAULT group default qlen 1000
61: foo@enp66s0f1: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 9000 qdisc noqueue state UP mode DEFAULT group default qlen 1000

While in VPP this is not so, there each individual interface and
sub-interface stands for itself. I think the proper fix here is to walk
all sub-interfaces when a phy changes, and force a sync of those from
VPP to LCP as well. I'll do that in a followup commit so it's easier to
roll back.

CMakeLists.txt

@@ -44,6 +44,7 @@ add_vpp_plugin(lcpng_if
   lcpng_if_api.c
   lcpng_if_cli.c
   lcpng_if_node.c
+  lcpng_if_sync.c
 
   API_FILES
   lcpng_if.api