zlib: fix gzip member head/buffer boundary issue

Make sure that, even if an `inflate()` call only sees the first
few bytes of a following gzip member, all members are decompressed
and part of the full output.

Adds tests for the special case that the first `inflate()` call
receives only the first few bytes of a second gzip member but
not the whole header (or even just the magic bytes).

This is a backport of #5883 and contains additional changes to
make sure that the behaviour on encountering trailing garbage
remains the same (namely to silently discard it if one full member
has already been decompressed).

PR-URL: https://github.com/nodejs/node/pull/5973


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