netdev: Fully handle netdev lifecycle through refcounting.

netdev: Fully handle netdev lifecycle through refcounting.

This builds on earlier work that implemented netdev object refcounting.
However, rather than requiring explicit create and destroy calls,
these operations are now performed automatically based on the referenece
count.  This is important because in certain situations it is not
possible to know whether a netdev has already been created.  A
workaround existed (which looked fairly similar to this paradigm) but
introduced it's own issues.  This simplifies and unifies the API.