classifier: Add support for conjunctive matches.

classifier: Add support for conjunctive matches.

A "conjunctive match" allows higher-level matches in the flow table, such
as set membership matches, without causing a cross-product explosion for
multidimensional matches.  Please refer to the documentation that this
commit adds to ovs-ofctl(8) for a better explanation, including an example.

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