cksum: add --algorithm option to select digest mode

cksum: add --algorithm option to select digest mode

* src/digest.c: Organize HASH_ALGO_CKSUM to be table driven,
and amalgamate all digest algorithms.
(main): Parse all options if HASH_ALGO_CKSUM, and disallow
--tag, --zero, and --check with the traditional bsd, sysv, and crc
checksums for now.
* src/local.mk: Reorganize to include all digest modules in cksum.
* tests/misc/cksum-a.sh: Add a new test.
* tests/misc/b2sum.sh: Update to default to checking with cksum,
as b2sum's implementation diverges a bit from the others.
* tests/local.mk: Reference the new test.
* doc/coreutils.texi (cksum invocation): Adjust the summary to
identify the new mode, and document the new --algorithm option.
* man/cksum.x: Adjust description to be more general.
* man/*sum.x: Add [See Also] section referencing cksum(1).
* NEWS: Mention the new feature.