Allow %start after the first rule.

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.