In iof, decode messages before printing them

[legoscia]

Use of_msg_lib:decode to decode records into proplists before printing
them. This has the advantage that the field names are readily visible.

For example, before this change, the output of iof:flows() looked like this:

{ofp_flow_stats_reply,[],
    [{ofp_flow_stats,0,8,404216000,1000,0,0,[],
         <<0,0,0,0,0,0,0,10>>,
         255,20188,
         {ofp_match,
             [{ofp_field,openflow_basic,in_port,false,<<0,0,0,1>>,undefined}]},
         [{ofp_instruction_apply_actions,2,
              [{ofp_action_output,16,2,no_buffer}]}]},
     {ofp_flow_stats,0,8,360038000,1000,0,0,[],
         <<0,0,0,0,0,0,0,10>>,
         237,18614,
         {ofp_match,
             [{ofp_field,openflow_basic,in_port,false,<<0,0,0,2>>,undefined}]},
         [{ofp_instruction_apply_actions,2,
              [{ofp_action_output,16,1,no_buffer}]}]}]}

With this change, the output looks like this:

  {flow_stats_reply,8,
      [{flags,[]},
       {flows,
           [[{table_id,0},
             {duration_sec,50758},
             {duration_nsec,927230000},
             {priority,1000},
             {idle_timeout,0},
             {hard_timeout,0},
             {flags,[]},
             {cookie,<<0,0,0,0,0,0,0,10>>},
             {packet_count,0},
             {byte_count,0},
             {match,[{in_port,<<0,0,0,2>>}]},
             {instructions,[{apply_actions,[{output,1,no_buffer}]}]}],
            [{table_id,0},
             {duration_sec,50758},
             {duration_nsec,971925000},
             {priority,1000},
             {idle_timeout,0},
             {hard_timeout,0},
             {flags,[]},
             {cookie,<<0,0,0,0,0,0,0,10>>},
             {packet_count,0},
             {byte_count,0},
             {match,[{in_port,<<0,0,0,1>>}]},
             {instructions,[{apply_actions,[{output,2,no_buffer}]}]}]]}]}

(the data is not identical in the two examples)