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.
Move yield to the tests and enable for FreeBSD

Move the definition of git_thread_yield() to the test which needs it and
add the correct definition for it for FreeBSD and derivatives.

Original patch adding FreeBSD and derivatives by @jacquesg.
Merge pull request #2460 from libgit2/cmn/sched-yield

Move yield to the tests and enable for FreeBSD
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'