[8u] 8217676: Upgrade libpng to 1.6.37

[Zhengyu Gu]

Hi Andrew,

> The license patch seems to miss some header lines when compared with
> 11u, and also removes a section about trademarks and export control that
> isn't altered in the 11u version. How was it applied?
> 
> I've attached what I get from manually applying the 11u patch to
> THIRD_PARTY_README. This diffs cleanly against libpng.md in 11u.
> 

Updated all THIRD_PARTY_README files with your patch.

http://cr.openjdk.java.net/~zgu/JDK-8217676-8u/v2/

Retested with jdk_imageio.

Thanks,

-Zhengyu

> Thanks,
>