Don't let -Wnone disable -Werror.

Discussed at
<http://lists.gnu.org/archive/html/bison-patches/2010-08/msg00009.html>.
* NEWS (2.5): Document.
* src/getargs.c (flags_argmatch): Accept a new argument that
specifies what flags "all" and thus "none" affect.
(FLAGS_ARGMATCH): Update flags_argmatch invocation.
* tests/input.at (-Werror is not affected by -Wnone and -Wall):
New test group.
(cherry picked from commit dab9663283437092d17af8ede75f3aff33380d53)