and png_convert_to_rfc1123(). (Frank Busse).

The 16-bit cases still use "L" as we don't have a 16-bit test system.

to conditions where types that are 32 bits in the GCC 32-bit
world (uLong and png_size_t) become 64 bits in the 64-bit
world.  This produces potential truncation errors which the
compiler correctly flags.

and fixed spelling (truely -> truly) in comments.

and fix some other small typos.

minor change from the previous (unreleased) ABI/API to hide the exact value
used for Macs - it's not a good idea to embed this in the ABI!