perf config: Introduce perf_config_set class

perf config: Introduce perf_config_set class

This infrastructure code was designed for upcoming features of
'perf config'.

That collect config key-value pairs from user and system config files
(i.e. user wide ~/.perfconfig and system wide $(sysconfdir)/perfconfig)
to manage perf's configs.

Reviewed-by: Masami Hiramatsu <mhiramat@kernel.org>
Signed-off-by: Taeung Song <treeze.taeung@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1460620401-23430-2-git-send-email-treeze.taeung@gmail.com
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>