Add Enchant support to ispell.el (Bug#17742)

Add Enchant support to ispell.el (Bug#17742)

* lisp/textmodes/ispell.el (ispell-program-name): Add “enchant”.
(ispell-really-enchant): Add variable.
(ispell-check-version): If using Enchant, check it’s new enough (at
least 1.6.1).  (Like the ispell check, this is absolute: cannot work
without.)
(ispell-enchant-dictionary-alist): Add variable.
(ispell-find-enchant-dictionaries): Add function, based on
ispell-find-aspell-dictionaries.
(ispell-set-spellchecker-params): Allow dictionary auto-detection for
Enchant, and call ispell-find-enchant-dictionaries to find them.  Use
old ispell name to locale mapping code for Enchant too.
(ispell-send-replacement): Make it work with Enchant.