png updated to 1.6.0

[Thomas Klausner <wiz%NetBSD.org@localhost>]

Hi!

I've upgraded png to 1.6.0. The update is minor compared to 1.4 -> 1.5 :)

One difference that might hit some packages: png.h included string.h,
but does not do that any longer. (I.e., you'll see missing prototype
warnings/errors for memset or getenv or a host of other functions.)

I fixed the packages I noticed that were affected by this. Let me
know if you find any other breakage introduced by this update.

Thanks,
 Thomas