Improve ispell.el word completion handling.

Improve ispell.el word completion handling.

* ispell.el (ispell-alternate-dictionary): Use file-readable-p.
  Return nil if no word-list is found at default locations.
 (ispell-complete-word-dict): Default to nil.
 (ispell-command-loop): Use 'word-list' when using lookup-words.
 (lookup-words): Use ispell-complete-word-dict or
 ispell-alternate-dictionary.  Check for word-list availability
 and handle errors if needed with better messages (Bug#6539).
 (ispell-complete-word): Use ispell-complete-word-dict or
 ispell-alternate-dictionary.