(assert_format): Do not bail out if several

--format arguments are given. This is a common case when
TAR_OPTIONS are used.
(decode_options): New option --show-defaults displays the
compiled-in defaults.

Use POSIX format if no --format option was given and
--pax-option was specified.

Do not allow to use --pax-option unless the archive format is
set to POSIX (or reading subcommand is requested).