perf tools: Add file_only config option to strlist

If strlist_config.dirname is present, the strlist__new() tries to load
stirngs from dirname/list file first but if it failes it falls back to
add 'list' as string.  But sometimes it's not desired so adds new
file_only field to prevent it.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
Cc: Andi Kleen <andi@firstfloor.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Jiri Olsa <jolsa@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Wang Nan <wangnan0@huawei.com>
Link: http://lkml.kernel.org/r/1452334589-8782-2-git-send-email-namhyung@kernel.org


[ Add documentation for strlist_config::file_only, in the struct definition */
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>