all: be less strict about usage if POSIX 2008

all: be less strict about usage if POSIX 2008

sort, tail, and uniq now support traditional usage like 'sort +2'
and 'tail +10' on systems conforming to POSIX 1003.1-2008 and later.
* NEWS: Document this.
* doc/coreutils.texi (Standards conformance, tail invocation)
(sort invocation, uniq invocation, touch invocation):
Document new behavior, or behavior's dependence on POSIX 1003.1-2001.
* src/sort.c (struct keyfield.traditional_used):
Rename from obsolete_used, since implementations are now allowed
to support it.  All uses changed.
(main): Allow traditional usage if _POSIX2_VERSION is 200809.
* src/tail.c (parse_obsolete_option): Distinguish between
traditional usage (which POSIX 2008 and later allows) and obsolete
(which it still does not).
* src/uniq.c (strict_posix2): New function.
(main): Allow traditional usage if _POSIX2_VERSION is 200809.
* tests/misc/tail.pl: Test for new behavior.