Register for operstate changes in OS network ports

OF port has a flag `link_down` that indicates that a physical link is
not present in the port. We assume that the OF port is up (i.e. this flag
is not set) only if the operstate of the port is `up`.

More about the operstate can be found here:
https://www.kernel.org/doc/Documentation/networking/operstates.txt
Handle messages from netlink indicating physical port state change

Netlink notifies OF port about `operstate` changes of the underlying
physical port. The OF port sets its set appropriately and notifies
a controller sending port status message.