Increase accuracy of tan near multiples of pi/4.

code/irrat-dd.lisp
o Make REDUCE-ARG return an extra result.  Since the reduction returns
  3 double's, return a double-double result and the third double
  result for extra accuracy
o Update dd-%tan and dd-%%tan to take the extra arg.
o Add new constant dd-pi/4-lo.
o Increase accuracy of tan by using the relationship
  tan(pi/4-y)=(1-tan(y))/(1+tan(y)).

tests/trig.lisp:
o Update the allowed error threshold for two tests to reflect the
  increased accuracy.