manuals: Remove references to libpngpf(3); update links

From libpng-1.5.0 onwards, the private libpng functions are no longer
accessible to the libpng users, and the content of libpngpf(3) is no
longer relevant.

Update links, introducing the new canonical location of the libpng
source repository:
https://github.com/pnggroup/libpng
2 files changed