cksum: add support for --algorithm=crc32b

cksum: add support for --algorithm=crc32b

  $ echo -n '123456789' | cksum --raw -a crc32b | basenc --base16
  CBF43926

* bootstrap.conf: Explicitly depend on the crc module.
* doc/coreutils.texi (cksum): Add "crc32b" as an argument to -a.
* src/cksum.c (crc32b_sum_stream): A new function similar to
crc_sum_stream, but which does not include the length in
the CRC calculation.
* src/cksum.h: Add crc32b_sum_stream prototype.
* src/digest.c: Add "crc32b" as an argument to -a.
* tests/cksum/cksum.sh: Refactor to test both crc and crc32b.
* tests/cksum/cksum-a.sh: Add "crc32b" case.
* tests/cksum/cksum-base64.pl: Likewise.
* tests/misc/read-errors.sh: Likewise.
* NEWS: Mention the new feature.