Introduce general-purpose ways to wait for dpif and netdev changes.

Introduce general-purpose ways to wait for dpif and netdev changes.

The dpif and netdev code has had various ways to check for changes to
dpifs and netdevs over the course of Open vSwitch development.  All of
these have been thus far fairly specific to the Linux implementation.  This
commit is the start of a more general API for watching for such changes.
The dpif-related parts seem fairly mature and so they are documented,
the netdev parts will probably need to change somewhat and so they are
not documented yet.