[libpng16] Update documentation about sCAL support.
[libpng16] Imported from libpng-1.6.9.tar
[libpng16] Bump version to 1.6.10beta01
[libpng16] Update ANNOUNCE file about new location of gpg signatures.
[libpng16] Backport recent changes from libpng-1.7.0beta30 and beta31.