ls: use the POSIX date style when the locale does not specify one

ls: use the POSIX date style when the locale does not specify one

Previously we defaulted to "long-iso" format in locales without
specific format translations, like the en_* locales for example.
This reverts part of commit 6837183d, 08-11-2005, "ls ... acts like
--time-style='posix-long-iso' if the locale settings are messed up"
* src/ls.c (decode_switches): Only use the ISO format when specified.
* NEWS: Mention the change in behavior.
Reported by Daniel Qarras at http://bugzilla.redhat.com/525134