More accurate incomplete-gamma function, add debugging to lentz, and

some random clean ups.

o Add *DEBUG-CF-EVAL* to enable debugging prints in LENTZ.
o Modify LENTZ to terminate with an error if *MAX-CF-ITERATIONS* is
  reached.
o Modify LENTZ to return the function value, the number of iterations,
  and the number of times a zero value had to be replaced.
o Adjust cf-incomplete-gamma and cf-incomplete-gamma-tail not to
  signal overflow prematurely when calculating z^a*exp(-z).
o Fix doc bug in reference for continued fraction for (original)
  cf-incomplete-gamma.
o Add new version of cf-incomplete-gamma using a different continued
  fraction.  This appears to converge faster and to be more accurate
  than the original, especially for points near the negative real
  axis.