Add -Wother so -Wnone suppresses all warnings.

Reported by George Neuner at
<http://lists.gnu.org/archive/html/bug-bison/2010-08/msg00002.html>.
* NEWS (2.5): Document.
* THANKS (George Neuner): Add.
* doc/bison.texinfo (Bison Options): Document.
* src/complain.c, src/complain.h
(warn_at, warn_at_indent, warn): Suppress warning if -Wno-other.
(midrule_value_at): New warning function, similar to yacc_at in
that it's controlled by its own warning category.
* src/getargs.c (warnings_flag): Initialize to warnings_other.
(warnings_args, warnings_types): Add entry for warnings_other.
(usage): Update.
* src/getargs.h (enum warnings): Add entry for warnings_other.
* src/gram.c (grammar_rules_useless_report): If -Wno-other, then
don't print useless rules.
* src/reader.c (symbol_should_be_used): Rather than adjusting the
return value based on whether midrule value warnings are enabled,
accept a new parameter for telling the caller whether true is
being returned for a potential midrule warning.
(grammar_rule_check): Use midrule_value_at for midrule value
warnings, and continue to use warn_at for all other warnings.  Let
them check whether the warnings are enabled.
* tests/local.at (AT_BISON_CHECK): Update documentation.
(AT_BISON_CHECK_NO_XML): Check that -Wnone and --warnings=none
disable all warnings exercised in the test suite.

doc/bison.texinfo

@@ -8426,10 +8426,16 @@ These warnings are not enabled by default since they sometimes prove to
 be false alarms in existing grammars employing the Yacc constructs
 @code{$0} or @code{$-@var{n}} (where @var{n} is some positive integer).
 
-
 @item yacc
 Incompatibilities with POSIX Yacc.
 
+@item other
+All warnings not categorized above.  These warnings are enabled by default.
+
+This category is provided merely for the sake of completeness.  Future
+releases of Bison may move warnings from this category to new, more specific
+categories.
+
 @item all
 All the warnings.
 @item none