(rounding_driver): New function for systematic support of

(rounding_driver): New function for systematic support of
2-argument rounding functions, so that `floor' isn't the only one
that supports 2 arguments.
(Fceiling, Ffloor, Fround, Ftruncate): Use it.
(ceiling2, floor2, round2, truncate2, double_identity): New functions.
(syms_of_floatfns): Define ceiling, round, and truncate even if
LISP_FLOAT_TYPE is not defined.