Use a GHashTable instead of a GArray to construct the list of
@interfaces. Also, use NMCListElem instead of GList. With this,
the runtime is O(n*log(n)) instead of O(n^2).

I belive, we should take care that all our code has a reasonable
runtime complexity, even in common use-cases the number of elements
is small. This is not about performace, because likely we expect few
entries anyway, and the direct GArray implementation is likely faster
in those cases. It's about using the data structure that best suits the
access pattern.

The log(n) part comes from sorting the keys. I also believe we should
always aim for a stable behavior. When sending the D-Bus request to
resolved, the order of elements should be in ~some~ defined order.