multistart: turn start symbols into rules on $accept

Now that the parser can read several start symbols, let's process
them, and create the corresponding rules.

* src/parse-gram.y (grammar_declaration): Accept a list of start symbols.
* src/reader.h, src/reader.c (grammar_start_symbol_set): Rename as...
(grammar_start_symbols_set): this.

* src/reader.h, src/reader.c (start_flag): Replace with...
(start_symbols): this.
* src/reader.c (grammar_start_symbols_set): Build a list of start
symbols.
(switching_token, create_start_rules): New.
(check_and_convert_grammar): Use them to turn the list of start
symbols into a set of rules.
* src/reduce.c (nonterminals_reduce): Don't complain about $accept,
it's an internal detail.
(reduce_grammar): Complain about all the start symbols that don't
derive sentences.

* src/symtab.c (startsymbol, startsymbol_loc): Remove, replaced by
start_symbols.
symbols_pack): Move the check about the start symbols
to...
* src/symlist.c (check_start_symbols): here.
Adjust to multiple start symbols.
* tests/reduce.at (Empty Language): Generalize into...
(Bad start symbols): this.

src/reader.h

@@ -38,7 +38,18 @@ typedef struct merger_list
 void free_merger_functions (void);
 extern merger_list *merge_functions;
 
-void grammar_start_symbol_set (symbol *sym, location loc);
+/* List of the start symbols.  */
+extern symbol_list *start_symbols;
+
+/* Fetch (or create) a token "YY_PARSE_foo" for start symbol "foo".
+
+   We don't use the simple "YY_FOO" because (i) we might get clashes
+   with some of our symbols (e.g., cast => YY_CAST), and (ii) upcasing
+   introduces possible clashes between terminal FOO and nonterminal
+   foo.  */
+symbol *switching_token (const symbol *start);
+
+void grammar_start_symbols_set (symbol_list *syms);
 
 void grammar_current_rule_begin (symbol *lhs, location loc,
                                  named_ref *lhs_named_ref);