ofp-util: Provide bucket list helper functions

This is in preparation for supporting the bucket commands of
(draft) Open Flow 1.5 group mod messages.

Also document ofputil_bucket_check_duplicate_id() for good measure.

ONF-JIRA: EXT-350
Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>