zlib: Move to a new zlib package

zlib is a standalone third-party component, and deserves its own
package rather than living in runtime. For example, this will make
future security updates less invasive. This also means there's no
dependency on runtime for ports that just require zlib, which is
useful for service jails.

Approved by: re (cperciva)
MFC after: 3 days
Reviewed by: bapt, emaste
Sponsored by: https://www.patreon.com/bsdivy
Differential Revision: https://reviews.freebsd.org/D53058

(cherry picked from commit 924c0c60d4aeaa474fb028118ca82d505166b4ab)
(cherry picked from commit a02e78a2c93e23d9bc2d8587c6809d7bdd9d07e7)