spell-fu lacks support for multiple dictionaries, affix expansion, and
many non-English dictionaries, so I've added back flyspell support to
the spell module, but opt-in, because it is still the significantly
slower option and spell-fu may support them one day.
If not, tlikonen/wcheck-mode is another alternative to consider.
Fixes#3813
Spell-fu is significantly faster, but does produce more false
positives (more faces must be added to spell-fu-faces-exclude to reduce
these).
Unfortunately, there is no fancy "correction" interface for spell-fu
yet, so we'll have to resort to ispell-word (on z=) for now.
