Until now, all implemented NMPObject types have an ifindex field (from
links, addresses, routes, qdisc to tfilter).

The NMPObject structure contains a union of all available types, that
makes it easier to down-case from an NMPObject pointer to the actual
content.

The "object" field of NMPObject of type NMPlatformObject is the lowest
common denominator.

We will add NMPlatformRoutingRules (for policy routing rules). That type
won't have an ifindex field.

Hence, drop the "ifindex" field from NMPlatformObject type. But also add
a new type NMPlatformObjWithIfindex, that can represent all types that
have an ifindex.