[libpng17] Renamed contrib/tools/png-fix-too-far-back.c to pngfix.c and revised

it to check all compressed chunks known to libpng.
[libpng17] Revised pngfix.c to use PNG_U32() instead of PNG_CHUNK()
[libpng17] Updated documentation to show default behavior of benign errors

correctly.
[libpng17] Added perfect hash code generation for lists of PNG chunks. This is

a work in progress; checked in for use in pngfix.c
[libpng17] Ported ARM no-read patch from libpng16.
[libpng17] Ported pngfix.c patches from libpng16.
Missing grammatical changes and update to png chunks list.

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