Rewrite to perform directory traversal using nftw.

Rewrite to perform directory traversal using nftw.

Include "dirname.h", "ftw.h", and "quotearg.h".
(AUTHORS): Add self.
(opt_one_file_system): Move global into `main'.
(path, xstat, exit_status): Remove declarations.
(arg_length, suffix_length): New globals.
(G_fail): New global, sort of like the old `exit_status'.
(IS_FTW_DIR_TYPE): Define.
(print_only_size): New function.
(process_file): New function.
(str_init, ensure_space, str_copyc, str_concatc): Remove functions.
(str_trunc, pop_dir, count_entry): Likewise.
(du_files): Rewrite to use nftw.