Log Message:
Changes 1.7.73:
* Restored calls to png_set_crc_action() which had been removed from
  version 1.7.72 for some testing and inadvertently not restored.
* Changed "fix" internal variable name to "salvage" (still \ 
set with "-fix")
* Added code to fix/salvage PNG with "bad adaptive filter value" error.
* Avoid calculating CRC during compression trials except for the last trial,
  when the output is actually written.
* Fixed a bug with reducing 16-bit images to 8-bit using "-reduce" option.