cksum: add --raw option to output a binary digest

cksum: add --raw option to output a binary digest

--raw output is the most composable format, and also is a
robust way to discard the file name without parsing (escaped) output.

Examples:

  $ cksum --raw -a crc "$afile" | basenc --base16
  4ACFC4F0

  $ cksum --raw -a crc "$afile" | basenc --base2msbf
  01001010110011111100010011110000

  $ cksum --raw -a sha256 "$bfile" | basenc --base32
  AAAAAAAADHLGRHAILLQWLAY6SNH7OY5OI2RKNQLSWPY3MCUM4JXQ====

* doc/coreutils.texi (cksum invocation): Describe the new feature.
* src/digest.c (output_file): Inspect the new RAW_DIGEST global,
and output the bytes directly if set.
* src/cksum.c (output_crc): Likewise.
* src/sum.c (output_bsd, output_sysv): Likewise.
* tests/misc/cksum-raw.sh: A new test.
* tests/local.mk: Reference the new test.
* NEWS: Mention the new feature.