Allow %start after the first rule.

* src/reader.c (grammar_current_rule_begin): Don't set the start symbol
when parsing the first rule.
(check_and_convert_grammar): Search for it here after all grammar
declarations have been parsed.  Skip midrules, which have dummy LHS
nonterminals.
* src/symtab.c (symbol_is_dummy): New function.
* src/symtab.h (symbol_is_dummy): Declare it.
* tests/input.at (%start after first rule): New test.

src/symtab.h

@@ -133,6 +133,9 @@ symbol *symbol_get (const char *key, location loc);
    Its name cannot conflict with the user's names.  */
 symbol *dummy_symbol_get (location loc);
 
+/** Is this a dummy nonterminal?  */
+bool symbol_is_dummy (const symbol *sym);
+
 /** Declare the new symbol \c sym.  Make it an alias of \c symval.  */
 void symbol_make_alias (symbol *sym, symbol *symval, location loc);