Add option to not compute or check check values.

Add option to not compute or check check values.

The undocumented (except in these commit comments) function
inflateValidate(strm, check) can be called after an inflateInit(),
inflateInit2(), or inflateReset2() with check equal to zero to
turn off the check value (CRC-32 or Adler-32) computation and
comparison. Calling with check not equal to zero turns checking
back on. This should only be called immediately after the init or
reset function. inflateReset() does not change the state, so a
previous inflateValidate() setting will remain in effect.

This also turns off validation of the gzip header CRC when
present.

This should only be used when a zlib or gzip stream has already
been checked, and repeated decompressions of the same stream no
longer need to be validated.