Folding procedures now reference configurable list

This allows users to define or override folding operations for any major or
minor mode.

This implementation does change the original functionality slightly:
Previously, hs-minor-mode was automatically enabled when folding, and ifdef
folding was enabled under c/c++ major modes.  I find this approach to be
both too magical and too rude---I do not want a folding operation to modify
my modes and (to make matters worse) potentially kick off hooks.
zO mapping for recursive fold opening

I did not yet add recursive closing beacuse we do not yet have the concept
of a `foldlevel'.
Folding support for outline and org modes