pngunknown: check user callback behavior
Previous versions compiled if SAVE_UNKNOWN was not available but did nothing
since the callback was never implemented.

Signed-off-by: John Bowler <jbowler@acm.org>
Merge with 1.7 version, back port 1.7 improvements/fixes

Signed-off-by: John Bowler <jbowler@acm.org>