* src/scan-gram.l: No longer "parse" things after `%union' until

`{'.  Rather, return a single "%union" token.
No longer make symbols: return strings, and leave the conversion
to symbols to the parser.
(SC_PRE_CODE, token_type): Remove.
* src/parse-gram.y (%union): New field `character'.
Sort tokens.
(CHAR): New token.
(ID, ID_COLON): Now that the scanner no longer makes them
identifiers, adjust all uses to invoke symbol_get.
(id_colon): New, wraps the conversion from string to symbol.
(%union): Accept a possible union_name.
(symbol): Now can be a char.
* data/c.m4 (b4_union_name): Leave a default value.
* data/glr.c, data/yacc.c: Use it.