nx-match: Add support for multiple OXM field assignments for one field.

nx-match: Add support for multiple OXM field assignments for one field.

actset_output, to be added in an upcoming commit, has one OXM assignment
in OpenFlow 1.3 and another one in OpenFlow 1.5.  This commit allows both
of them to be supported in appropriate OpenFlow versions.

This feature is difficult to test on its own, so the same commit that adds
actset_output support also tests this feature.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>