zlib: add support for concatenated members

According to the spec gzipped archives can contain more than one
compressed member. Previously Node's gzip implementation would only
unzip the first member and throw away the rest of the compressed data.
Issue #4306 is an example of this occurring in daily use.

Fixes: https://github.com/nodejs/node/issues/4306
PR-URL: https://github.com/nodejs/node/pull/5120


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: James M Snell <jasnell@gmail.com>