Restore extra help output.

* src/tar.c (tar_list_quoting_styles): Change first argument to
struct obstack.
(format_default_settings): New function.
(show_default_settings)
(show_default_settings_fs): Removed.
(tar_help): Removed.
(tar_help_filter): New function.
(argp): Set help_filter.
(parse_opt): Fix error message.

gnulib.modules

@@ -5,6 +5,7 @@ alloca
 argmatch
 argp
 argp-version-etc
+asprintf
 backupfile
 canonicalize
 closeout