tee: treat '-' operand as file name as mandated by POSIX

tee: treat '-' operand as file name as mandated by POSIX

Since v5.2.1-1247-g8dafbe5, tee(1) treated '-' as stdout while POSIX
explicitly requires to treat this as a file name.  Revert this change,
as the interleaved output - due to sending another copy of input to
stdout - is not considered to be useful.  Discussed in
http://lists.gnu.org/archive/html/coreutils/2015-02/msg00085.html

* src/tee.c (tee_files): Remove the special handling for "-" operands.
(usage): Remove the corresponding sentence.
* doc/coreutils.texi (common options): Remove the "tee -" example.
(tee invocation): Document that tee(1) now treats "-" as a file name.
* tests/misc/tee.sh: Add a test case for "tee -".
While at it, re-indent the above multi-argument processing case and
extend that to 13 operands, as POSIX mandates that, too.
* tests/misc/tee-dash.sh: Remove now-obsolete test.
* tests/local.mk (all_tests): Remove the above test.
* NEWS (Changes in behavior): Mention the change.