1.0.32.20: bug fixes in unibyte external formats

1.0.32.20: bug fixes in unibyte external formats

* handling undefined codepoints:

  There's a difference between "unassigned codepoint", represented as
  (<code> nil), and "codepoint mapping to character with that codepoint",
  represented by the absence of an entry in the exceptions list.  Simply
  testing for trueness of (cadr (assoc <code> exceptions)) isn't good
  enough; test for trueness of (cdr (assoc ...)) and use the car if so.

* undefined codepoints as the first exception:

  If the first exception to code<->byte is an undefined character, as for
  example in iso-8859-8, the lowest-code exception was computed wrongly,
  leading to incorrect encoding.