png-1.5 and some BSD fixes

Hi!

png-1.5 hides structure members from public view. For this reason,
some code doesn't compile any more.

Patches against 2.8.6 are attached; I've checked that the current
development version still has this problem (patch-ae, patch-af).

While checking this, I noted that some other patches from pkgsrc
haven't been included upstream, so I attach these as well.

patch-ab: the charset name is ucs-4 with a "-" on NetBSD.
patch-ac: endian support for Dragonfly BSD

Please include the patches!

Thanks,
 Thomas