Update to zlib 1.3.1

Update to zlib 1.3.1

Since we already pulled in all fixes, this only bumps version/magic numbers
and copyright years. The fixes relevant to OpenBSD are:

- Fix bug in inflateSync() for data held in bit buffer
- Add LIT_MEM define to use more memory for a small deflate speedup
- Add bounds checking to ERR_MSG() macro, used by zError()
- Various portability and appearance improvements