date: use extended format timezone for --iso-8601
* src/date.c (main): Use %:z rather than %z with --iso-8601
as the standard states to consistently use extended format.
Note either format can be parsed by date.
* tests/misc/date.pl: Adjust accordingly.
* doc/coreutils.texi (du invocation): Clarify that "iso"
time styles are only similar to ISO-8601.
(ls invocation): Likewise.
(date invocation): Adjust the comment stating
that only --rfc-3339 output can be parsed by date(1).
* NEWS: Mention the change in behavior.
Reported at http://bugs.debian.org/799479