Gzip: use zlib to write header and trailer. When nginx is used with zlib patched with [1], which provides integration with the future IBM Z hardware deflate acceleration, it ends up computing CRC32 twice: one time in hardware, which always does this, and one time in software by explicitly calling crc32(). crc32() calls were added in changesets 133:b27548f540ad ("nginx-0.0.1- 2003-09-24-23:51:12 import") and 134:d57c6835225c ("nginx-0.0.1- 2003-09-26-09:45:21 import") as part of gzip wrapping feature - back then zlib did not support it. However, since then gzip wrapping was implemented in zlib v1.2.0.4, and it's already being used by nginx for log compression. This patch replaces hand-written gzip wrapping with the one provided by zlib. It simplifies the code, and makes it avoid computing CRC32 twice when using hardware acceleration. [1] https://github.com/madler/zlib/pull/410