perf daemon: Add 'list' command

Add a 'list' command to display all running sessions.  It's the default
command if no other command is specified.

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

List sessions:

  # perf daemon
  [771394:daemon] base: /opt/perfdata
  [771395:cycles] perf record -m 10M -e cycles --overwrite --switch-output -a
  [771396:sched] perf record -m 20M -e sched:* --overwrite --switch-output -a

List sessions with more info:

  # perf daemon -v
  [771394:daemon] base: /opt/perfdata
    output:  /opt/perfdata/output
  [771395:cycles] perf record -m 10M -e cycles --overwrite --switch-output -a
    base:    /opt/perfdata/session-cycles
    output:  /opt/perfdata/session-cycles/output
  [771396:sched] perf record -m 20M -e sched:* --overwrite --switch-output -a
    base:    /opt/perfdata/session-sched
    output:  /opt/perfdata/session-sched/output

The 'output' file is perf record output for specific session.

Note you have to stop all running perf processes manually at this point,
stop command is coming in following patches.

Committer notes:

Fixup union initialization to overcome this in multiple older systems:

  22    15.74 debian:8                      : FAIL gcc version 4.9.2 (Debian 4.9.2-10+deb8u2)

    builtin-daemon.c: In function 'send_cmd_list':
    builtin-daemon.c:1386:2: error: missing initializer for field 'csv_sep' of 'struct <anonymous>' [-Werror=missing-field-initializers]
      };
      ^
    builtin-daemon.c:641:8: note: 'csv_sep' declared here
       char csv_sep;
            ^
    cc1: all warnings being treated as errors

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/20210208200908.1019149-11-jolsa@kernel.org


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