Qualifying nexthops via BGP does not properly work since nexthops need

Qualifying nexthops via BGP does not properly work since nexthops need
to be rechecked when a BGP route is added (or changed).

We need to revalidate nexthops on inserts (kroute_insert) and
on change (krX_change but only for AID_INET and AID_INET6 -- no nexthops
in the other tables) the nexthop needs to be updated.

Only validate nexthops if 'nexthop qualify via bgp' is enabled. For route
changes the code can depend on the F_NEXTHOP flag.

Fix for: https://github.com/openbgpd-portable/openbgpd-portable/issues/81
OK tb@