perf config: Validate config variable arguments before trying use them

You can show the values for several config items as below:

    # perf config report.queue-size call-graph.record-mode

but it is necessary to more precisely check arguments, before passing
them to show_spec_config().  This validation function would be also used
when parsing config key-value pairs arguments in the near future.

Committer notes:

Testing it:

  $ perf config bla.
  The config variable does not contain a variable name: bla.
  $ perf config .bla
  The config variable does not contain a section name: .bla
  $ perf config bla.bla
  $

Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Tested-by: Arnaldo Carvalho de Melo <acme@redhat.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Nambong Ha <over3025@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wang Nan <wangnan0@huawei.com>
Cc: Wookje Kwon <aweee0@gmail.com>
Link: http://lkml.kernel.org/r/1478241862-31230-4-git-send-email-treeze.taeung@gmail.com
[ Fix some spelling errors ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

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