Add -Wconflicts-sr and -Wconflicts-rr.

Thus, conflict reports are now affected by -Werror and -Wnone
(unless %expect or %expect-rr is specified).  Reported by George
Neuner at
<http://lists.gnu.org/archive/html/bug-bison/2010-08/msg00002.html>.
* NEWS (2.5): Document.
* doc/bison.texinfo (Bison Options): Document.
* src/complain.c, src/complain.h (set_warning_issued): Export
function.
* src/conflicts.c (conflicts_print): Suppress conflict report
based on -Wno-conflicts-sr and -Wno-conflicts-rr, and treat
conflicts as errors if -Werror.
* src/getargs.c (warnings_flag): Initialize with
warnings_conflicts_sr and warnings_conflicts_rr as well.
(warnings_args, warnings_types): Add entries for
warnings_conflicts_sr and warnings_conflicts_rr.
(usage): Update.
* src/getargs.h (enum warnings): Add entries for
warnings_conflicts_sr and warnings_conflicts_rr.
* tests/conflicts.at (-W versus %expect and %expect-rr): New test
group.
* tests/local.at (AT_BISON_CHECK_NO_XML): Update now that the
conflict report can produce a "warnings being treated as errors"
message.  Also, check that stderr is now fully scrubbed by -Wnone
when the exit status is 0.

src/getargs.h

@@ -123,7 +123,9 @@ enum warnings
     warnings_error            = 1 << 0, /**< Warnings are treated as errors.  */
     warnings_midrule_values   = 1 << 1, /**< Unset or unused midrule values.  */
     warnings_yacc             = 1 << 2, /**< POSIXME.  */
-    warnings_other            = 1 << 3, /**< All other warnings.  */
+    warnings_conflicts_sr     = 1 << 3, /**< S/R conflicts.  */
+    warnings_conflicts_rr     = 1 << 4, /**< R/R conflicts.  */
+    warnings_other            = 1 << 5, /**< All other warnings.  */
     warnings_all              = ~warnings_error /**< All above warnings.  */
   };
 /** What warnings are issued.  */