tools lib: Add bitmap_scnprintf function

Add support to print bitmap list. Code mostly taken from kernel's
bitmap_list_string.

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/1470074555-24889-3-git-send-email-jolsa@kernel.org


[ s/bitmap_snprintf/bitmap_scnprintf/g as it is a scnprintf wrapper, having the same semantics wrt return value ]
Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>