Now that we return a struct from get_config(), we can have system-wide
properties returned.
Let it count and cache the number of valid iface_datas.
Currently that is not yet used, but it will be.
(cherry picked from commit a3cd66d3fa)
Returning a struct seems easier to understand, because then the result
is typed.
Also, we might return additional results, which are system wide and not
per-interface.
(cherry picked from commit 323e182768)
