ofp_sendto should return the number of bytes sent #250

It appears that ofp_send/sendto returns 0 if the packet is sent. This diverges from the normal socket send/sendto calls in that they return the number of bytes sent. Not sure if that is intentional or not as the apis don't seem to be documented as to what they supposed to return. However, given that there is netwrap stuff, it seems like it is intended to mimic the same api.