ofproto/ofproto-dpif.c: Re-implement the ofproto/trace command

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.

ofproto/trace [datapath] odp_flow [-generate|packet]
ofproto/trace bridge br_flow [-generate|packet]

Also, this patch updates the tests and the manpages accordingly.

Co-authored-by: Ben Pfaff <blp@nicira.com>
Signed-off-by: Alex Wang <alexw@nicira.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>