get-config() gives a NMCSProviderGetConfigResult structure, and the
main part of data is the GHashTable of MAC addresses and
NMCSProviderGetConfigIfaceData instances.
Let NMCSProviderGetConfigIfaceData also have a reference to the MAC
address. This way, I'll be able to create a (sorted) list of interface
datas, that also contain the MAC address.
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.
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.
