ofproto: Represent flow cookie changes as operations too.

ofproto: Represent flow cookie changes as operations too.

An upcoming commit will add support for monitoring changes to the flow
table.  This feature wants to be able to report changes to flow cookies,
as well as to other properties of a flow.  Until now, however, a flow_mod
that modifies only the flow's cookie is treated as a special case that does
not go through the ofoperation mechanism.  That makes it harder to report
flow cookie-only changes (it would require an additional special case in
the reporting mechanism) so this commit changes cookie-only changes to
go through ofoperations.

The bulk of this change is to change the meaning of ofoperation's 'ofpacts'
member so that a NULL value indicates that the flow's actions are not
changing.  Otherwise a flow-cookie only change would still require copying
and then freeing all the actions, which seems like a waste.

Signed-off-by: Ben Pfaff <blp@nicira.com>