Back out change to unknown handling #defines
The change breaks 'NOREAD' builds.

Signed-off-by: John Bowler <jbowler@acm.org>
[libpng16] Reverted the change to unknown handling #defines; the change

breaks 'NOREAD' builds.
[libpng16] Added #ifdef PNG_READ_SUPPORTED/endif around the

png_chunk_unknown_handling() function, and restored the unknown-chunk
handling code from libpng-1.6.6.

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Glenn R-P
Merge branch 'libpng16' of /home/jbowler/src/libpng/libpng into libpng16
Attempted merge.

Conflicts:
	png.c
	pngrutil.c
Fix compilation with NO_SAVE_CHUNKS+NO_WRITE
The combination removes support for saving a list of unknown chunks, which,
counter-intuitively, WRITE support turns on because of the API to write unknown
chunks.  This combination of options did not previously work although it is
clearly useful in the READ case where the application might wish to handle all
unknown chunks directly (without a save.)

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