Use new @ escapes consistently.

Use new @ escapes consistently.
(b4_input_suffix, b4_output_parser_suffix, b4_output_parser_name,
b4_output_header_suffix, b4_output_header_name, b4_header_guard):
Remove, since they couldn't handle arbitrary characters in file
names.

Remove use of "#ifdef b4_header_guard", since it
mishandled funny characters in file names, and anyway it isn't
needed any more.

(YYSTYPE_IS_TRIVIAL): Define when the .h file would.