perf config: Show the configuration when no arguments are provided

More convenient thah having to recall what letter is about
showing/listing/dumping the configuration, i.e. no arguments means
-l/--list:

  # perf config
  llvm.dump-obj=true
  trace.default_events=/home/acme/git/perf/tools/perf/examples/bpf/augmented_raw_syscalls.o
  trace.show_zeros=yes
  trace.show_duration=no
  # perf config -l
  llvm.dump-obj=true
  trace.default_events=/home/acme/git/perf/tools/perf/examples/bpf/augmented_raw_syscalls.o
  trace.show_zeros=yes
  trace.show_duration=no
  # perf config -h

   Usage: perf config [<file-option>] [options] [section.name[=value] ...]

      -l, --list            show current config variables
          --system          use system config file
          --user            use user config file

  #

Cc: Adrian Hunter <adrian.hunter@intel.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Luis Cláudio Gonçalves <lclaudio@redhat.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Taeung Song <treeze.taeung@gmail.com>
Cc: Wang Nan <wangnan0@huawei.com>
Link: https://lkml.kernel.org/n/tip-z2n63avz6tliqb5gmu4l1dti@git.kernel.org


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