perf evlist: Add --trace-fields option to show trace fields

To use dynamic sort keys, it might be good to add an option to see the
list of field names.

  $ perf evlist -i perf.data.sched
  sched:sched_switch
  sched:sched_stat_wait
  sched:sched_stat_sleep
  sched:sched_stat_iowait
  sched:sched_stat_runtime
  sched:sched_process_fork
  sched:sched_wakeup
  sched:sched_wakeup_new
  sched:sched_migrate_task
  # Tip: use 'perf evlist --trace-fields' to show fields for tracepoint events

  $ perf evlist -i perf.data.sched --trace-fields
  sched:sched_switch: trace_fields: prev_comm,prev_pid,prev_prio,prev_state,next_comm,next_pid,next_prio
  sched:sched_stat_wait: trace_fields: comm,pid,delay
  sched:sched_stat_sleep: trace_fields: comm,pid,delay
  sched:sched_stat_iowait: trace_fields: comm,pid,delay
  sched:sched_stat_runtime: trace_fields: comm,pid,runtime,vruntime
  sched:sched_process_fork: trace_fields: parent_comm,parent_pid,child_comm,child_pid
  sched:sched_wakeup: trace_fields: comm,pid,prio,success,target_cpu
  sched:sched_wakeup_new: trace_fields: comm,pid,prio,success,target_cpu
  sched:sched_migrate_task: trace_fields: comm,pid,prio,orig_cpu,dest_cpu

Committer notes:

For another file, in verbose mode:

  # perf evlist -v --trace-fields
  sched:sched_switch: type: 2, size: 112, config: 0x10b, { sample_period, sample_freq }: 1, sample_type: IP|TID|TIME|CPU|PERIOD|RAW, disabled: 1, inherit: 1, mmap: 1, comm: 1, task: 1, sample_id_all: 1, exclude_guest: 1, mmap2: 1, comm_exec: 1, trace_fields: prev_comm,prev_pid,prev_prio,prev_state,next_comm,next_pid,next_prio
  #

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Frederic Weisbecker <fweisbec@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1452125549-1511-5-git-send-email-namhyung@kernel.org


[ Replaced 'trace_fields=' with 'trace_fields: ' to make the output consistent in -v mode ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>