perf record: Add 'snapshot' control command

Add 'snapshot' control command to create an AUX area tracing snapshot
the same as if sending SIGUSR2. The advantage of the FIFO is that access
is governed by access to the FIFO.

Example:

  $ mkfifo perf.control
  $ mkfifo perf.ack
  $ cat perf.ack &
  [1] 15235
  $ sudo ~/bin/perf record --control fifo:perf.control,perf.ack -S -e intel_pt//u -- sleep 60 &
  [2] 15243
  $ ps -e | grep perf
   15244 pts/1    00:00:00 perf
  $ kill -USR2 15244
  bash: kill: (15244) - Operation not permitted
  $ echo snapshot > perf.control
  ack
  $

Signed-off-by: Adrian Hunter <[email protected]>
Acked-by: Alexey Budankov <[email protected]>
Acked-by: Jiri Olsa <[email protected]>
Tested-by: Arnaldo Carvalho de Melo <[email protected]>
Cc: Andi Kleen <[email protected]>
Cc: Namhyung Kim <[email protected]>
Link: http://lore.kernel.org/lkml/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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