ofproto/ofproto-dpif.c: Re-implement the ofproto/trace command
Since the use of single datapath, all bridges belonging to the same type of
datapath will use the same (single) datapath. This causes confusion in the
current 'ofproto/trace' command. Especially, when given the unrelated
'bridge' and 'in_port' combination, the current implementation will still
be able to process and give misleading output. Thusly, this patch changes
the 'ofproto/trace' command syntax to formats shown as follow.