Make Fzlib_decompress_region always call the change hooks in a balanced way.

Make Fzlib_decompress_region always call the change hooks in a balanced way.

This means there will be exactly one call of each of before- and
after-change-functions, regardless of whether or not the decompression
succeeds, and these calls will refer to corresponding buffer regions.

src/decompress.c (struct decompress_unwind_data): add a new field, orig.
(unwind_decompress): Use del_range_2 and update_compositions in place of
del_range, to avoid unwanted change hook calls.  Call signal_after_change for
the failed case.
(Fzlib_decompress_region): Call modify_txt for a before-change-functions.  Set
the new field orig of unwind_data to the region's start.  Use del_range_2 and
update_compositions in place of del_range to avoid unwanted change hook
calls.  Call signal_after_change for the successful case.