cksum: support digest detection for tagged format

cksum: support digest detection for tagged format

Support `cksum --check FILE` without having to specify a digest
algorithm, allowing for more generic file check instructions.
This also supports mixed digest checksum files, supporting
more robust multi digest checks.

* src/digest.c (algorithm_from_tag): A new function to
identify the digest algorithm from a tagged format line.
(split3): Set the algorithm depending on tag, and update
the expected digest length accordingly.
* tests/misc/cksum-c.sh: Add a new test.
* tests/local.mk: Reference the new test.
* tests/misc/md5sum.pl: Adjust to more generic error.
* tests/misc/sha1sum.pl: Likewise.
* doc/coreutils.texi (md5sum invocation): Mention the new -c feature.
* NEWS: Mention the new feature.