Gzip: support for a zlib variant from Intel. A zlib variant from Intel as available from https://github.com/jtkukunas/zlib uses 64K hash instead of scaling it from the specified memory level, and also uses 16-byte padding in one of the window-sized memory buffers, and can force window bits to 13 if compression level is set to 1 and appropriate compile options are used. As a result, nginx complained with "gzip filter failed to use preallocated memory" alerts. This change improves deflate_state allocation detection by testing that items is 1 (deflate_state is the only allocation where items is 1). Additionally, on first failure to use preallocated memory we now assume that we are working with the Intel's modified zlib, and switch to using appropriate preallocations. If this does not help, we complain with the usual alerts. Previous version of this patch was published at http://mailman.nginx.org/pipermail/nginx/2014-July/044568.html. The zlib variant in question is used by default in ClearLinux from Intel, see http://mailman.nginx.org/pipermail/nginx-ru/2017-October/060421.html, http://mailman.nginx.org/pipermail/nginx-ru/2017-November/060544.html.