fixed-format floating point printing: zero and scaling factors

fixed-format floating point printing: zero and scaling factors

  Now that FLONUM-TO-DIGITS handles zero, we need to check against
  zero before adding in the scaling factor.

  Also make sure not to print extra digits when E is negative.

  Adjust FORMAT-AUX-EXP to not print the extra-zero: FLONUM-TO-STRING
  provides it now.

  Fixes lp#811386.