Require default %destructor/%printer to be declared using

%symbol-default instead of an empty symbol list, and start working on
new per-type %destructor/%printer.  Discussed at
<http://lists.gnu.org/archive/html/bison-patches/2006-09/msg00007.html>.
* NEWS (2.3+): Add %symbol-default to example.
* bison.texinfo (Freeing Discarded Symbols): Likewise.
(Bison Symbols): Add entry for %symbol-default.
* src/parse-gram.y (PERCENT_SYMBOL_DEFAULT): New token.
(generic_symlist, generic_symlist_item): New nonterminals for creating
a list in which each item is a symbol, semantic type, or
%symbol-default.
(grammar_declaration): Use generic_symlist in %destructor and %printer
declarations instead of symbols.1 or an empty list.
(symbol_declaration, precedence_declaration, symbols.1): Update actions
for changes to symbol_list.
* src/reader.c: Update for changes to symbol_list.
* src/scan-code.l: Likewise.
* src/scan-gram.l: Scan new PERCENT_SYMBOL_DEFAULT token.
* src/symlist.c, src/symlist.h: Extend such that a list node may
represent a semantic type or a %symbol-default in addition to just an
ordinary symbol.  Add switched functions for setting %destructor's and
%printer's.
* tests/actions.at, tests/input.at: Add %symbol-default to all default
%destructor/%printer declarations.

src/scan-gram.l

@@ -193,6 +193,7 @@ splice	 (\\[ \f\t\v]*\n)*
   "%skeleton"             	    return PERCENT_SKELETON;
   "%start"                	    return PERCENT_START;
   "%start-header"                   return PERCENT_START_HEADER;
+  "%symbol-default"                 return PERCENT_SYMBOL_DEFAULT;
   "%term"                 	    return PERCENT_TOKEN;
   "%token"                	    return PERCENT_TOKEN;
   "%token"[-_]"table"     	    return PERCENT_TOKEN_TABLE;