Improve const use in zlib-using code

Improve const use in zlib-using code

If we define ZLIB_CONST before including zlib.h, zlib augments some
interfaces with const decorations.  By doing that we can keep our own
interfaces cleaner and can remove some unconstify calls.

ZLIB_CONST was introduced in zlib 1.2.5.2 (17 Dec 2011).  When
compiling with older zlib releases, you might now get compiler
warnings about discarding qualifiers.

CentOS 6 has zlib 1.2.3, but in 8e278b6576, we removed support for the
OpenSSL release in CentOS 6, so it seems ok to de-support the zlib
release in CentOS 6 as well.

Reviewed-by: Tristan Partin <tristan@neon.tech>
Discussion: https://www.postgresql.org/message-id/flat/33462926-bb1e-7cc9-8d92-d86318e8ed1d%40eisentraut.org