Update zlib to 1.2.8
Merge branch 'cmn/zlib-128' into cmn/zlib-update

Conflicts:
	deps/zlib/crc32.c
	deps/zlib/crc32.h
	deps/zlib/zconf.h
Merge branch 'cmn/zlib-update' into cmn/update-zlib
zlib: add a few missing defines
zlib: get rid of gz*
zlib: get rid of compress.c and uncompr.c
zlib: disable warning 4142 on MSVC

This is about benign redefinition of types. We're not interested in it.
Fix git_cred_ssh_interactive_callback signature
Include libssh2.h before git2.h (transport.h)
Merge pull request #2461 from jacquesg/ssh2-warnings

Fix SSH2 warnings
Merge remote-tracking branch 'origin/cmn/update-zlib'
ssh: provide a factory function for setting ssh paths

git allows you to set which paths to use for the git server programs
when connecting over ssh; and we want to provide something similar.

We do this by providing a factory function which can be set as the
remote's transport callback which will set the given paths upon
creation.