cksum: use --tag format by default

cksum: use --tag format by default

This format is a better default, since it results in simpler usage,
as you don't need to specify --tag on generation or -a on
checking invocations.  Also it's a more general format supporting
mixed and length adjusted digests.

* doc/coreutils.texi (cksum invocation): Document a new --untagged
option, to use the older coreutils format.
(md5sum invocation): Mention that cksum doesn't support --tag.
* src/digest.c: Adjust cksum(1) to default to --tag,
and accept the new --untagged option.
* tests/misc/b2sum.sh: Adjust accordingly.
* tests/misc/cksum-a.sh: Likewise.
* tests/misc/cksum-c.sh: Likewise.