[libpng16] Changed png_convert_to_rfc_1123() to png_convert_to_rfc_1123_buffer()

in the manual, to reflect the change made in libpng-1.6.0.
Updated README file to reflect that direct access to the info struct and
png struct are not permitted since libpng-1.5.0.
[libpng16] Imported from libpng-1.6.14rc02.tar
[libpng16] Bump version to 1.6.14rc03
[libpng16] Minor editing of README