zlib: make constants keep readonly

In zlib module, a dozen constants were exported to user land,
If user change the constant, maybe lead unexcepted error.

Make them readonly and freezon.

PR-URL: https://github.com/iojs/io.js/pull/1361


Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: Shigeki Ohtsu <ohtsu@iij.ad.jp>