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>

tools/perf/Documentation/perf-config.txt

@@ -8,7 +8,7 @@ perf-config - Get and set variables in a configuration file.
 SYNOPSIS
 --------
 [verse]
-'perf config' -l | --list
+'perf config' [<file-option>] -l | --list
 
 DESCRIPTION
 -----------
@@ -21,6 +21,14 @@ OPTIONS
 --list::
 	Show current config variables, name and value, for all sections.
 
+--user::
+	For writing and reading options: write to user
+	'$HOME/.perfconfig' file or read it.
+
+--system::
+	For writing and reading options: write to system-wide
+	'$(sysconfdir)/perfconfig' or read it.
+
 CONFIGURATION FILE
 ------------------
 
@@ -30,6 +38,10 @@ The '$HOME/.perfconfig' file is used to store a per-user configuration.
 The file '$(sysconfdir)/perfconfig' can be used to
 store a system-wide default configuration.
 
+When reading or writing, the values are read from the system and user
+configuration files by default, and options '--system' and '--user'
+can be used to tell the command to read from or write to only that location.
+
 Syntax
 ~~~~~~