- don't even bother to look into the platform cache, but use
  if_indextoname() / if_nametoindex(). In most cases, we obtained
  the ifindex/ifname not from the platform cache in the first
  place. Hence, there is a race, where the interface might not
  exist.
  However, try to process events of the platform cache, hoping
  that the cache contains an interface for the given ifindex/ifname.

- let set_ip_ifindex() and set_ip_iface() both return a boolean
  value to indicate whether a ip-interface is set or not. That is,
  whether we have a positive ip_ifindex. That seems more interesting
  information, then to return whether anything changed.

- as before, set_ip_ifindex() can only clear an ifindex/ifname,
  or error out without doing anything. That is different from
  set_ip_iface(), which will also set an ifname if no ifindex
  can be resolved. That is curreently ugly, because then ip-ifindex
  and ip-iface don't agree. That shall be improved in the future
  by:
  - trying to set an interface that cannot be resolved shall
    lead to a disconnect in any case.
  - we shall make less use of the ip-iface and rely more on the
    ifindex.