zlib: detect gzip files when using unzip*

Detect whether a gzip file is being passed to `unzip*` by
testing the first bytes for the gzip magic bytes, and setting
the decompression mode to `GUNZIP` or `INFLATE` according to
the result.

This enables gzip-only features like multi-member support
to be used together with the `unzip*` autodetection support
and thereby makes `gunzip*` and `unzip*` return identical
results for gzip input again.

Add a simple test for checking that features specific to
`zlib.gunzip`, notably support for multiple members, also work
when using `zlib.unzip`.

PR-URL: https://github.com/nodejs/node/pull/5884


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