perf daemon: Add 'signal' command

Allow the 'perf daemon' to send SIGUSR2 to all running sessions or just
to a specific session.

Example:

  # cat ~/.perfconfig
  [daemon]
  base=/opt/perfdata

  [session-cycles]
  run = -m 10M -e cycles --overwrite --switch-output -a

  [session-sched]
  run = -m 20M -e sched:* --overwrite --switch-output -a

Start the daemon:

  # perf daemon start

Send signal to all running sessions:

  # perf daemon signal
  signal 12 sent to session 'cycles [773738]'
  signal 12 sent to session 'sched [773739]'

Or to specific one:

  # perf daemon signal --session sched
  signal 12 sent to session 'sched [773739]'

And verify signals were delivered and perf.data dumped:

  # cat /opt/perfdata/session-cycles/output
  rounding mmap pages size to 32M (8192 pages)
  [ perf record: dump data: Woken up 1 times ]
  [ perf record: Dump perf.data.2021010220382490 ]

  # car /opt/perfdata/session-sched/output
  rounding mmap pages size to 32M (8192 pages)
  [ perf record: dump data: Woken up 1 times ]
  [ perf record: Dump perf.data.2021010220382489 ]
  [ perf record: dump data: Woken up 1 times ]
  [ perf record: Dump perf.data.2021010220393745 ]

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Alexei Budankov <abudankov@huawei.com>
Cc: Ian Rogers <irogers@google.com>
Cc: Ingo Molnar <mingo@kernel.org>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Michael Petlan <mpetlan@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: https://lore.kernel.org/r/20210208200908.1019149-12-jolsa@kernel.org


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>