ofproto-dpif: Introduce "internal flows" for handling flow table misses.

ofproto-dpif: Introduce "internal flows" for handling flow table misses.

The ofproto-dpif implementation of "facet"s requires a facet to be
associated with an OpenFlow rule.  Until now, this meant that packets
that miss in the OpenFlow table (and thus didn't have OpenFlow rules)
couldn't be set up as facets and thus couldn't be installed in the
kernel.  This commit changes that, by introducing "internal" OpenFlow
rules to associate with such packets.

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