tsort now works more like the traditional UNIX tsort. Before it would

tsort now works more like the traditional UNIX tsort.  Before it would
exit when it found a loop.  Now it continues and outputs all items.

(exit_status): New variable.
(loop): New varibale.
(count_items, scan_zeroes): Change return type to int.
(detect_loop): Complete rewrite to correctly implement detection
of loops.  Also change return type to int.
(recurse_tree): Stop if ACTION returns non-zero.  This involves
changing the return type of this function and ACTION to int.
(walk_tree): Change return type of ACTION to int.
(tsort): Continue sort after a loop has been detected (and
broken).  Set exit_status to 1 if a loop was detected.
(main): Use exit_status to determine exit code.