Invalidate next-hop based on neighbor cache state

This feature invalidates a static route based on the reachability of the next-hop in the neighbor cache when the validate-next-hop command is enabled within the static-route-entry>next-hop context for an IPv6 static route.

In this case, when the Neighbor Cache entry for next-hop is INVALID or not populated, the static route must remain invalid/inactive. When an NC entry for next-hop is populated based on a neighbor advertisement received, or periodic traffic destined for it and the usual NS/NA procedure, the static route becomes valid/active and is installed.