cksum: support transparent emulation of older utils

cksum: support transparent emulation of older utils

Support -b, --binary, and -t, --text
to allow full emulation of older utilities with:
  exec cksum -a $algo --untagged "$@"
Note this would diverge from OpenBSD's support of cksum -b.

* src/digest.c: Change -b to mean --binary, not --base64 in all cases.
Accept -b and -t in all cases.  Keep --binary and --text undocumented
for cksum.
* tests/cksum/cksum-base64.pl: s/-b/--base64/.
* tests/cksum/cksum-a.sh: Ensure cksum supports -b and -t appropriately.
* NEWS: Mention the change in behavior.