See infback9.h for what this is and how to use it.

Original code available here (zlib 1.3):
https://github.com/madler/zlib/tree/09155eaa2f9270dc4ed1fa13e2b4b2613e6e4851/contrib/infback9

This copy of zlib 1.3's unsupported infback9 code has been modified as follows:
* Added this notice to README.
* Added "LICENSE" (the license section of zlib.h) from the main zlib code.
* Added a stripped-down copy of zutil.h sufficient to build infback9 against.
* Added include guards to headers.
* Fixed a fall-through warning.
* Replaced references to zcalloc and zcfree with inline implementations. This
  is due to older versions of zlib seemingly not exposing these.
* Removed OF() wrapper for function declarations (unnecessary).
