pbrd: Pay attention to interface up/down events with nht

pbrd: Pay attention to interface up/down events with nht

When an interface goes up/down we need to pay attention to this
in PBR.  In the past we were relying *only* on the nht events
but this is not sufficient for cases where an interface is flapping
up and down.  If this is happening it could be happening fast enough
that zebra is not sending nht events because they are consolidated
into a single event from it's perspective and that is the right thing
to do.  This commit will allow us to back out commit:

0aaa722883245c2109d9856ca0656749860fc579

As that commit introduced extra processing in zebra that is actually
causing issues in other places.  The problem that commit was trying
to solve should have always been handled in pbrd instead of making
zebra do work that is unnatural to it's actual flow.

Signed-off-by: Donald Sharp <sharpd@nvidia.com>