(struct item, n_strings): Use size_t for sizes.

(struct item, n_strings): Use size_t for sizes.
(have_read_stdin, count_items, scan_zeros, detect_loop,
recurse_tree, walk_tree, tsort, main):
Use bool for booleans.
(exit_status): Remove.
(tsort): Return a success flag instead of storing into a global.
(main): Use it.