perf config: Add '--system' and '--user' options to select which config file is used

The '--system' option means $(sysconfdir)/perfconfig and '--user' means
$HOME/.perfconfig. If none is used, both system and user config file are
read.  E.g.:

    # perf config [<file-option>] [options]

    With an specific config file:

    # perf config --user | --system

    or both user and system config file:

    # perf config

Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Link: http://lkml.kernel.org/r/1455126685-32367-2-git-send-email-treeze.taeung@gmail.com


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