pngcrush-1.7.73 released

Fixed a bug with reducing 16-bit images to 8-bit using "-reduce" option
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.