[libpng17] Revised example.c to illustrate use of PNG_DEFAULT_sRGB and

PNG_GAMMA_MAC_18 as parameters for png_set_gamma().
[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.
[libpng17] Imported from libpng-1.7.0beta16.tar
[libpng17] Bump version to 1.7.0beta17
[libpng17] Port recent /*SAFE*/ comments and manual changes from libpng16