Switch to new packet-out format and add OFPP_IN_PORT.

Switch to new packet-out format and add OFPP_IN_PORT.

The original packet-out format allowed multiple actions to be specified for
buffered packets, but only a destination port for messages including data.
This change makes packet-out more consistent by allowing multiple actions
regardless of how the packet is stored.

This change also disallows sending packets through the incoming port without
explicitly using the OFPP_IN_PORT virtual port.