lib: add support for interface nexthops on nexthop groups

lib: add support for interface nexthops on nexthop groups

This patch adds support to nexthops of type NEXTHOP_TYPE_IFINDEX to
nexthop-groups. This should be especially useful when dealing with
p2p interfaces like tunnels that don't have IP addresses assigned
to them.

NOTE: nh->addr can be NULL now, so we should always perform a null
check before dereferencing this pointer.

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