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.

NEWS

@@ -309,6 +309,13 @@ Bison News
 
     bison -Wall,no-yacc gram.y
 
+*** The "none" category no longer disables a preceding "error":
+
+  For example, for the following command line, Bison now reports
+  errors instead of warnings for incompatibilities with POSIX Yacc:
+
+    bison -Werror,none,yacc gram.y
+
 * Changes in version 2.4.3 (2010-08-05):
 
 ** Bison now obeys -Werror and --warnings=error for warnings about