(isnan): Define even if LISP_FLOAT_TYPE is not defined, since fmod

(isnan): Define even if LISP_FLOAT_TYPE is not defined, since fmod
might need it.
(fmod): Ensure that the magnitude of the result does not exceed that
of the divisor, and that the sign of the result does not disagree with
that of the dividend.  This does not yield a
particularly accurate result, but at least it will be in the
range promised by fmod.