perf tools: Add ping control command

Adding control 'ping' command to detect if perf is up and
control interface is operational.

It will be used in following daemon patches to synchronize
with record session - when control interface is up and
running, we know that perf record is monitoring and ready
to receive signals.

Example session:

  terminal 1:
    # mkfifo control ack
    # perf record --control=fifo:control,ack

  terminal 2:
    # echo ping > control
    # cat ack
    ack

Signed-off-by: Jiri Olsa <jolsa@kernel.org>