perf config: Write a config file just once

Currently set_config() can be repeatedly called for each input config on
the below case:

  $ perf config kmem.default=slab report.children=false ...

But it's a waste, so only once write a config file gathering all given
config key=value pairs.

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/1504754331-9776-1-git-send-email-treeze.taeung@gmail.com


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