In some cases we need to be able to locate a device by its IPv4 address.
There doesn't seem to be an easy way to do this on Linux, so we iterate
through all devices until it's found. The caller can provide a hint as
to the device, so subsequent checks can be quicker.
This checkin also adds nodev versions of functions to lookup ARP entries
and the IPv4 address of a device.
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.
