cksum: operate in binary mode only

cksum: operate in binary mode only

This only practically matters on windows.
But given there are separate text handling options in cygwin,
keep the interface simple, and avoid exposing the
confusing binary/text difference here.

* doc/coreutils.texi (md5sum invocation): Mention that
--binary and --text are not supported by the cksum command.
* src/digest.c: Set flag to use binary mode by default.
(output_file): Don't distinguish text and binary modes with
' ' and '*', and just use ' ' always.