[libpng16] Change "png-fix-too-far-back" to "pngfix" inside pngfix.c

for error messages, etc.
[libpng16] Bump version to 1.6.3beta10
[libpng16] Updated documentation to show default behavior of benign errors

correctly.
[libpng16] Only compile ARM code when PNG_READ_SUPPORTED is defined.
[libpng16] Fixed undefined behavior in contrib/tools/pngfix.c and added new

strip option. pngfix relied on undefined behavior and even a simple change from
gcc to g++ caused it to fail.  The new strip option 'unsafe' has been
implemented and is the default if --max is given.  Option names have
been clarified, with --strip=transform now stripping the bKGD chunk,
which was stripped previously with --strip=unused.
Added all documented chunk types to pngpriv.h
[libpng16] Unified pngfix.c source with libpng17.