[RFC] flow: Support OF1.5+ (draft) actset_output field.

This field allows a flow table to match on the output port currently in the
action set.

As-is, this patch has a number of deficiencies.  I will fix them; I am
posting for now because I feel like I have sat on this patch series for
long enough.

Known problems:

    * When the action set contains a group action and an output action,
      actset_output should become OFPP_UNSET.

    * Missing documentation.

    * Missing tests.

    * OF1.3 and OF1.4 should use ONFOXM_ET_ACTSET_OUTPUT; OF1.5+ should use
      OXM_OF_ACTSET_OUTPUT.  The current patch uses the former for all
      versions.

ONF-JIRA: EXT-233
Signed-off-by: Ben Pfaff <blp@nicira.com>