net-lib: add configured_ifaces()

net-lib: add configured_ifaces()

configured_ifaces is a function that returns the names of each interface
that the user wanted configured.

Currently, this is accomplished by reading the list from
/tmp/net.ifaces. But if we want to allow the user to specify an
interface by its MAC address or IP or something, we need a function that
will read the cache and convert the MACs etc. to names.

(Obviously this conversion only works once udev starts, so it will warn
you if you try it too early.)