Add intrinsic checksum support.

Currently uses CityHash64.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>
Import table-driven, software crc32c from FreeBSD.

Fix intrinsic checksum computation.  The checksum must be taken
EITHER at full length after the first read which fills the
checksum window or at the record length that is smaller.

Signed-off-by: Matt Benjamin <matt@linuxbox.com>