MFC r311285,312335: zlib 1.2.11.

MFC r311275: Restructure libz.

Restructure libz, place vendor files in contrib/zlib like other third
party software, this provides more standarized import workflow and
makes future upgrades easier.

The following files are new with this commit:

zconf.h.in
zlib.map
zlib.pc.in

They are not connected to build, but were kept in tree for reference
for future maintenance.

All our local trivial changes were applied to contrib/zlib, and the
contrib/zlib vendor source code is intended to 100% match lib/libz
before this commit.

MFC after: 2 weeks

Copy vendor/libz@250225 to vendor/zlib.

Vendor import of zlib 1.2.8.

Vendor import of zlib 1.2.7.

Vendor import of zlib 1.2.6.