cksum: use more accurate program name in single binary mode

cksum: use more accurate program name in single binary mode

Following commit v9.9-109-gcd5229222

* src/cksum.c (PROGRAM_NAME): In legacy mode, i.e., when built
as a single binary, set the PROGRAM_NAME dynamically based on
the selected digest type.  This is significant in --help
to ensure the correct texinfo node is referenced, and in
--version to ensure the correct utility name is output.