(ispell-local-dictionary-overridden): New var.

(ispell-local-dictionary-overridden): New var.
(ispell-local-dictionary): Doc fix.
(ispell-dictionary-alist): Don't include ispell-local-dictionary-alist.
Don't reinitialize at run time.  Don't defcustom.
All uses changed to append ispell-local-dictionary-alist,
or else check it first.
(ispell-current-dictionary): New variable for dictionary in use.
(ispell-dictionary): Now used only for global default.
(ispell-start-process): Set ispell-current-dictionary,
not ispell-dictionary.
(ispell-change-dictionary): Use this only for setting
user preferences.
(ispell-internal-change-dictionary): New function
to change the current dictionary in use.
(ispell-region, ispell-process-line, ispell-buffer-local-dict):
Use ispell-current-dictionary.
Handle ispell-local-dictionary-overridden.
(ispell-buffer-local-dict): Call ispell-internal-change-dictionary.