Extended types: Miscellaneous fixes of xt_bool

Extended types: Miscellaneous fixes of xt_bool

  •  Streamlined code and unified coding style.

  •  xt_bool_format() returns static strings, there is no need
     to duplicate them.

  •  xt_bool_parse() returns a more reasonable error message.

  •  Invalid formatting mode is an internal error, which should not
     be triggerable by the user, so ASSERT is enough.