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/complain.h

@@ -25,6 +25,13 @@
 extern "C" {
 # endif
 
+/** Record that a warning is about to be issued, and treat it as an
+    error if <tt>warnings_flag & warnings_error</tt>.  This is exported
+    only for the sake of Yacc-compatible conflict reports in conflicts.c.
+    All other warnings should be implemented in complain.c and should use
+    the normal warning format.  */
+void set_warning_issued (void);
+
 /** Informative messages, but we proceed.  Report iff
     <tt>warnings_flag & warnings_other</tt>.  */