(prot_a_bitstring.add_field): Store the field, and check for duplicates.
(prot_a_bitstring.check_implemented): New method.
(lexer.__init__): Read conftypes.tmp.
(lexer.toplevel_bye): Check Extended-Conf-Type.
(lexer.__parse_userdefined_bitstring): Be prepared that
	prot_a_bitstring.add_field can return an error message.