Improve const use in zlib-using code

Now that we have effectively de-supported CentOS 6, by removing support
for its OpenSSL version, I think we could also modernize the use of some
other libraries, such as zlib.

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); CentOS 6 has
zlib-1.2.3-29.el6.x86_64.

Note that if you use this patch and compile on CentOS 6, it still works,
you just get a few compiler warnings about discarding qualifiers. Old
environments tend to produce more compiler warnings anyway, so this
doesn't seem so bad.