Disable unset/unused mid-rule value warnings by default, and recognize

--warnings=midrule-values to enable them.  Discussed starting at
<http://lists.gnu.org/archive/html/help-bison/2006-10/msg00030.html>.
* NEWS (2.3a+): Mention.
* src/getargs.c, src/getargs.h (warnings_args, warnings_types, enum
warnings): Add entry for midrule-values subargument.
* src/reader.c (symbol_should_be_used): Don't return true just because
the value is a set/used mid-rule value unless
--warnings=midrule-values was specified.
* tests/input.at (Unused values, Unused values before symbol
declarations): Run tests with and without --warnings=midrule-values.

* src/reader.c (check_and_convert_grammar): Use symbol_list_free rather
than LIST_FREE directly.

NEWS

@@ -6,6 +6,26 @@ Changes in version 2.3a+ (????-??-??):
 * The -g and --graph options now output graphs in Graphviz DOT format,
   not VCG format.
 
+* Revised warning: unset or unused mid-rule values
+
+  Since Bison 2.2, Bison has warned about mid-rule values that are set but not
+  used within any of the actions of the parent rule.  For example, Bison warns
+  about unused $2 in:
+
+    exp: '1' { $$ = 1; } '+' exp { $$ = $1 + $4; };
+
+  Now, Bison also warns about mid-rule values that are used but not set.  For
+  example, Bison warns about unset $$ in the mid-rule action in:
+
+    exp: '1' { $1 = 1; } '+' exp { $$ = $2 + $4; };
+
+  However, Bison now disables both of these warnings by default since they
+  sometimes prove to be false alarms in existing grammars employing the Yacc
+  constructs $0 or $-N (where N is some positive integer).
+
+  To enable these warnings, specify the flag `--warnings=midrule-values' or
+  `-W', which is a synonym for `--warnings=all'.
+
 * Bison now recognizes two separate kinds of default %destructor's and
   %printer's: