Fixes for python HBAC bindings
    
    These changes were proposed during a review:
     * Change the signature of str_concat_sequence() to const char *
     * use a getsetter for HbacRule.enabled to allow string true/false and
       integer 1/0 in addition to bool
     * fix a minor memory leak (HbacRequest.rule_name)
     * remove overzealous discard consts