Currently 'perf ftrace' command allows selecting 'function_graph' or
'function', defaulting to 'function_graph'.

Add ftrace.tracer config option to select the default tracer:

    # cat ~/.perfconfig
    [ftrace]
        tracer = function

    # perf ftrace usleep 123456 | head -10
      <...>-14450 [002] d... 10089.284231: finish_task_switch <-__schedule
      <...>-14450 [002] .... 10089.284232: finish_wait <-pipe_wait
      <...>-14450 [002] .... 10089.284232: mutex_lock <-pipe_wait
      <...>-14450 [002] .... 10089.284232: _cond_resched <-mutex_lock

Committer notes:

Retesting it with invalid variables, invalid values for ftrace.tracer,
and a valid one:

  # cat ~/.perfconfig
  [ftrace]
        trace = function
  # perf ftrace usleep 1
  Error: wrong config key-value pair ftrace.trace=function
  # cat ~/.perfconfig
  [ftrace]
        tracer = functin
  # perf ftrace usleep 1
  Please select "function_graph" (default) or "function"
  Error: wrong config key-value pair ftrace.tracer=functin
  # cat ~/.perfconfig
  [ftrace]
        tracer = function
  # perf ftrace usleep 1 | head -5
          <idle>-0     [000] d...  3855.820847: switch_mm_irqs_off <-__schedule
           <...>-18550 [000] d...  3855.820849: finish_task_switch <-__schedule
           <...>-18550 [000] d...  3855.820851: smp_irq_work_interrupt <-irq_work_interrupt
           <...>-18550 [000] d...  3855.820851: irq_enter <-smp_irq_work_interrupt
           <...>-18550 [000] d...  3855.820851: rcu_irq_enter <-irq_enter
  #
Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1485862711-20216-3-git-send-email-treeze.taeung@gmail.com
[ Added missign space in error message, changed the logic to make it more compact and less error prone ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>