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 <[e-mail]>
Tested-by: Arnaldo Carvalho de Melo <[e-mail]>
Cc: Borislav Petkov <[e-mail]>
Cc: David Ahern <[e-mail]>
Cc: Jiri Olsa <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Link: http://lkml.kernel.org/r/20170217170018.GA15389@krava
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

tools/perf/Documentation/perf-record.txt

···
 
 -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

···
 
 	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,