[pngcrush] add comments about how to get more verbose help

Update zlib to version 1.2.11

[pngcrush] Update bundled libpng to version 1.6.26

[pngcrush] Fixed "nolib" build (bug report by Hanspeter Niederstrasser).

[pngcrush] Update changelog entry

Imported from pngcrush-1.8.6.tar

Imported from pngcrush-1.8.5.tar

Imported from pngcrush-1.8.4.tar

Imported from pngcrush-1.8.3.tar

[pngcrush] Update pngcrush to version 1.8.2

Fixed filesize reduction report when "-ow" option is used (Bug report #68).
When a single method is specified, turn -reduce off by default and skip