ispell.el: Make sure hunspell specific dict names are used for standard dicts when...

ispell.el: Make sure hunspell specific dict names are used for standard dicts when using hunspell.

(ispell-hunspell-dictionary-equivs-alist):
  New variable to map standard dict names to hunspell ones.

(ispell-set-spellchecker-params):
  Make sure specific dict names are used for standard dicts with hunspell.

  If a mapping is missing, skip that dictionary.

Fixes: debbugs:13460