pbrd: add support for interface nexthops

pbrd: add support for interface nexthops

Now that nexthop groups can contain interface nexthops, make the
necessary adjustments in pbrd to handle them appropriately.

For normal IP nexthops, pbrd uses the NHT callbacks to validate
these nexthops (i.e. check if they are reachable). NHT can't be
used for interface nexthops though. To work around this issue,
use the interface event callbacks from the zclient API to validate
interface nexthops (an interface nexthop is valid only if the
corresponding interface is up and running).

Signed-off-by: Renato Westphal <renato@opensourcerouting.org>