ofp-print: Print out_port field in "flow_mod"s.

Somehow we've overlooked this for a long time.  I guess that feature
doesn't get used very much.

Also updates the test output for OFPT_FLOW_MOD to include the out_port
output and adds out_port to the NXT_FLOW_MOD tests.

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