ofproto: Improve abstraction by using OpenFlow port numbers in interface.

ofproto: Improve abstraction by using OpenFlow port numbers in interface.

Until now, ofproto has used a mix of datapath and OpenFlow port numbers in
its client interface.  This commit changes it to use OpenFlow port numbers
exclusively, to raise the level of abstraction.

Most of this commit boils down to simple search-and-replace with a few
call to ofp_port_to_odp_port() sprinkled in.  The addition of ofproto_port
is one exception.  An ofproto_port is almost the same as a dpif_port; the
difference is just that its port number is an OpenFlow port number instead
of a datapath port number.