Remove some lint, found by gcc -W etc.

Remove some lint, found by gcc -W etc.

* src/common.h (label_notfound): New decl.
* src/buffer.c (set_volume_start_time, compress_type):
(guess_seekable_archive, open_compressed_archive, init_buffer):
(_flush_write, archive_is-dev, increase_volume_number):
(change_tape_menu, try_new_volume, add_chunk_header):
(multi_volume_sync):
Declare as 'static' if it's not exported.
Use function prototype (void) rather than old-style ().
* src/checkpoint.c (expand_checkpoint_string): Likewise.
* src/incremen.c (dirlist_replace_prefix, makedumpdir, read_incr_db_2):
Likewise.
* src/list.c (print_volume_label): Likewise.
* src/misc.c (normalize_filename_x): Likewise.
* src/names.c (make_name, free_name, check_name_alloc, name_next_elt):
Likewise.
* src/tar.c (tar_list_quoting_style, add_exclude_array):
(set_stat_signal): Likewise.
* src/transform.c (new_transform, _single_transform_name_to_obstack):
(_transform_name_to_obstack): Likewise.
* src/unlink.c (dunlink_alloc): Likewise.

* src/buffer.c (struct zip_magic): Use const when appropriate.
* src/incremen.c (obstack_code_rename, write_directory_file_entry):
Likewise.
* src/sparse.c (COPY_STRING): Likewise.
* src/system.c (dec_to_env, time_to_env, oct_to_env, str_to_env):
(chr_to_env): Likewise.
* src/tar.c (tar_list_quoting_style, set_stat_signal): Likewise.

* src/extract.c (extract_node): Don't return garbage.

* src/names.c: Remove old-style declarations of getgrnam etc.
All modern systems declare these, and it's not worth the hassle
of ignoring the warnings on modern systems for old-style decls.