Add support for listing and deleting entries based on an output port.

Add support for listing and deleting entries based on an output port.

To support this, an "out_port" field has been added to the "ofp_flow_mod",
"ofp_flow_stats_request", and "ofp_aggregate_stats_request" messages.  If an
"out_port" contains a value other than "OFPP_NONE", it introduces a constraint
when matching.  This constraint is that the rule must contain an output action
directed at that port.  Other constraints such as ofp_match structs and
priorities are still used; this is purely an *additional* constraint.  Note
that to get previous behavior, though, "out_port" must be set to "OFPP_NONE",
since "0" is a valid port id.  This only applies to the delete and
delete_strict flow mod commands; the field is ignored by add, modify, and
modify_strict.