perf script: Print comm, fork and exit events also

perf script: Print comm, fork and exit events also

If --show-task-events option is given, also print internal COMM, FORK
and EXIT events.  It would be helpful for debugging.

  $ perf script --show-task-events
  ...
         swapper     0 [009] 3350640.335261: sched:sched_switch: prev_comm=swapper/9
           sleep  9486 [009] 3350640.335509: PERF_RECORD_COMM: sleep:9486
           sleep  9486 [009] 3350640.335806: sched:sched_stat_runtime: comm=sleep pid=9486
         firefox  2635 [003] 3350641.275896: PERF_RECORD_FORK(2635:9487):(2635:2635)
         firefox  2635 [003] 3350641.275896: sched:sched_process_fork: comm=firefox pid=2635
           sleep  9486 [009] 3350641.336009: PERF_RECORD_EXIT(9486:9486):(9486:9486)

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Suggested-by: Frederic Weisbecker <fweisbec@gmail.com>
Reviewed-by: David Ahern <dsahern@gmail.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Namhyung Kim <namhyung.kim@lge.com>
Cc: Paul Mackerras <paulus@samba.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1385455873-25865-1-git-send-email-namhyung@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>