zebra: support nh resolution without a route

zebra: support nh resolution without a route

Start reorg of zebra nexthop-resolution so that we can use the
resolution logic for nexthop-groups as well as routes. Change
the signature of the core nexthop_active() api so that it does
not require a route-entry or route-node. Move some of the logic
around so that nexthop-specific logic is in nexthop_active(),
while route-oriented logic is in nexthop_active_check().

Signed-off-by: Mark Stapp <mjs@voltanet.io>