perf record: Add -a as default target

Running 'perf record' with no target (-a, -p, -t, etc) will now collect
system wide data.

Commiter notes:

Testing it:

  [root@jouet ~]# perf record
  ^C[ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 1.351 MB perf.data (366 samples) ]
  #

is equivalent to:

  # perf record -a
  ^C[ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 1.411 MB perf.data (978 samples) ]
  #

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/20170217170018.GA15389@krava
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

tools/perf/Documentation/perf-record.txt

@@ -157,7 +157,7 @@ OPTIONS
 
 -a::
 --all-cpus::
-        System-wide collection from all CPUs.
+        System-wide collection from all CPUs (default if no target is specified).
 
 -p::
 --pid=::

tools/perf/builtin-record.c

@@ -1677,8 +1677,10 @@ int cmd_record(int argc, const char **argv, const char *prefix __maybe_unused)
 
 	argc = parse_options(argc, argv, record_options, record_usage,
 			    PARSE_OPT_STOP_AT_NON_OPTION);
+
+	/* Make system wide (-a) the default target. */
 	if (!argc && target__none(&rec->opts.target))
-		usage_with_options(record_usage, record_options);
+		rec->opts.target.system_wide = true;
 
 	if (nr_cgroups && !rec->opts.target.system_wide) {
 		usage_with_options_msg(record_usage, record_options,