Be a bit more consistent about comparing to zero

* src/buffer.c (xclose, archive_is_dev, close_archive)
(write_fatal_details, init_volume_number)
(closeout_volume_number, new_volume, try_new_volume):
* src/checkpoint.c (format_checkpoint_string):
* src/compare.c (process_rawdata, diff_file, diff_dumpdir):
* src/create.c (create_archive, restore_parent_fd, dump_file0):
* src/delete.c (delete_archive_members):
* src/exclist.c (cvs_addfn):
* src/extract.c (set_mode, mark_after_links, delay_set_stat)
(repair_delayed_set_stat, make_directories, file_newer_p)
(maybe_recoverable, apply_nonancestor_delayed_set_stat)
(extract_dir, open_output_file, find_delayed_link_source)
(create_placeholder_file, extract_symlink, extract_node)
(extract_fifo, apply_delayed_link):
* src/incremen.c (update_parent_directory, scan_directory)
(read_obstack, read_incr_db_2, write_directory_file)
(try_purge_directory):
* src/map.c (map_read):
* src/misc.c (maybe_backup_file, undo_last_backup, chdir_do)
(tar_savedir):
* src/names.c (handle_file_selection_option, add_file_id)
(handle_option, read_next_name, add_hierarchy_to_namelist)
(collect_and_sort_names):
* src/system.c (run_decompress_program, dec_to_env, time_to_env)
(oct_to_env, str_to_env, chr_to_env, sys_exec_setmtime_script):
* src/tar.c (get_date_or_file, parse_default_options)
(decode_options, main):
* src/unlink.c (flush_deferred_unlinks):
* src/update.c (append_file):
* src/xattrs.c (xattrs__acls_set, xattrs_xattrs_set):
Prefer < 0 when looking at syscalls; prefer != 0 to nothing
when testing an integer in a boolean context.
This is for style, not substance; for example, it’s easier
to read ‘if (wordsplit (...) != WRDSE_OK) ...’ than
‘if (wordsplit (...)) ...’ if you don’t already know that
wordsplit returns an enum rather than bool.
* src/names.c (add_file_id, read_next_name, regex_usage_warning):
* src/transform.c (parse_xform_flags):
Return bool not int, possibly inverting sense so that true means OK.
All callers changed.
* src/tar.c (main): Report errno info if stdopen fails.

src/unlink.c

@@ -106,7 +106,7 @@ flush_deferred_unlinks (bool force)
 	      else
 		fname = p->file_name;
 
-	      if (unlinkat (chdir_fd, fname, AT_REMOVEDIR) != 0)
+	      if (unlinkat (chdir_fd, fname, AT_REMOVEDIR) < 0)
 		{
 		  switch (errno)
 		    {
@@ -132,7 +132,7 @@ flush_deferred_unlinks (bool force)
 	    }
 	  else
 	    {
-	      if (unlinkat (chdir_fd, p->file_name, 0) != 0 && errno != ENOENT)
+	      if (unlinkat (chdir_fd, p->file_name, 0) < 0 && errno != ENOENT)
 		unlink_error (p->file_name);
 	    }
 	  dunlink_reclaim (p);
@@ -166,7 +166,7 @@ flush_deferred_unlinks (bool force)
 	  else
 	    fname = p->file_name;
 
-	  if (unlinkat (chdir_fd, fname, AT_REMOVEDIR) != 0)
+	  if (unlinkat (chdir_fd, fname, AT_REMOVEDIR) < 0)
 	    {
 	      if (errno != ENOENT)
 		rmdir_error (fname);