Do not let internal JDK zlib symbols leak out of fastdebug libzip.so

[Martin Buchholz]

I have another iteration of this change
http://cr.openjdk.java.net/~martin/webrevs/openjdk8/hide-zlib/
that adds exciting new exception detail message for the InternalError I was
scratching my head about earlier.

-            msg = strm->msg;
+            msg = ((strm->msg != NULL) ? strm->msg :
+                   (ret == Z_VERSION_ERROR) ?
+                   "zlib returned Z_VERSION_ERROR: "
+                   "compile time and runtime zlib implementations differ" :
+                   (ret == Z_STREAM_ERROR) ?
+                   "inflateInit2 returned Z_STREAM_ERROR" :
+                   "unknown error initializing zlib library");