tools lib api fs: Add sysfs__read_str function

Adding sysfs__read_str function to ease up reading string files from
sysfs. New interface is:

  int sysfs__read_str(const char *entry, char **buf, size_t *sizep);

Signed-off-by: Jiri Olsa <jolsa@kernel.org>
Cc: David Ahern <dsahern@gmail.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/1455465826-8426-4-git-send-email-jolsa@kernel.org
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>