maint: use C99 initialization syntax for single-variable 'for' loops

maint: use C99 initialization syntax for single-variable 'for' loops

* src/chroot.c (parse_additional_groups): Declare variable in the 'for'
clause.
* src/comm.c (compare_files): Likewise.
* src/cp.c (re_protect): Likewise.
* src/csplit.c (save_buffer, find_line, write_to_file)
(split_file, main): Likewise.
* src/dd.c (apply_translations, translate_buffer)
(copy_with_block, main): Likewise.
* src/df.c (selected_fstype, excluded_fstype, filter_mount_list)
(last_device_for_mount, get_device, get_point, get_all_entries)
(main): Likewise.
* src/fmt.c (copy_rest, get_prefix, fmt_paragraph, put_paragraph)
(put_word): Likewise.
* src/fold.c (main): Likewise.
* src/head.c (elide_tail_bytes_pipe, main): Likewise.
* src/install.c (main): Likewise.
* src/join.c (prfields, join, main): Likewise.
* src/kill.c (list_signals): Likewise.
* src/ls.c (main, decode_switches, parse_ls_color, patterns_match):
Likewise.
* src/operand2sig.c (operand2sig): Likewise.
* src/pathchk.c (no_leading_hyphen, validate_file_name): Likewise.
* src/pr.c (char_to_clump): Likewise.
* src/printenv.c (main): Likewise.
* src/ptx.c (initialize_regex, digest_break_file)
(find_occurs_in_text, print_field): Likewise.
* src/remove.c (mark_ancestor_dirs): Likewise.
* src/seq.c (print_numbers): Likewise.
* src/shred.c (do_wipefd, main): Likewise.
* src/sort.c (cleanup, inittables, key_warnings, mergefps)
(check_ordering_compatibility, main): Likewise.
* src/split.c (closeout): Likewise.
* src/stat.c (find_bind_mount, print_it, format_to_mask): Likewise.
* src/stdbuf.c (set_program_path): Likewise.
* src/stty.c (apply_settings, display_changed, display_all)
(recover_mode, sane_mode): Likewise.
* src/system.h (stzncpy): Likewise.
* src/tail.c (pipe_lines): Likewise.
* src/tee.c (tee_files): Likewise.
* src/tr.c (look_up_char_class, get_spec_stats): Likewise.
* src/users.c (list_entries_users): Likewise.