Add support to detect changes to the daemon's config file triggering a
re-read of the configuration when that happens.

Use a inotify file descriptor plugged into the main fdarray object for
polling.

Example:

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

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

Starting the daemon:

  # perf daemon start

Check sessions:

  # perf daemon
  [772262:daemon] base: /opt/perfdata
  [772263:cycles] perf record -m 10M -e cycles --overwrite --switch-output -a

Change '-m 10M' to '-m 20M', and check daemon log:

  # tail -f /opt/perfdata/output
  [2021-01-02 20:31:41.234045] daemon started (pid 772262)
  [2021-01-02 20:31:41.235072] reconfig: ruining session [cycles:772263]: -m 10M -e cycles --overwrite --switch-output -a
  [2021-01-02 20:32:08.310137] reconfig: session 'cycles' killed
  [2021-01-02 20:32:08.310847] reconfig: ruining session [cycles:772338]: -m 20M -e cycles --overwrite --switch-output -a

And the session list:

  # perf daemon
  [772262:daemon] base: /opt/perfdata
  [772338:cycles] perf record -m 20M -e cycles --overwrite --switch-output -a

Note the changed '-m 20M' option is in place.
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/20210208200908.1019149-8-jolsa@kernel.orgSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>