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Move contrib/zlib to sys/contrib/zlib so that we can use it in kernel.

Description

Move contrib/zlib to sys/contrib/zlib so that we can use it in kernel.
This is a prerequisite of unifying kernel zlib instances.

Submitted by: Yoshihiro Ota <ota at j.email.ne.jp>
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D20191