sum: always output a file name if one passed

sum: always output a file name if one passed

Adjust to output the file name if any name parameter is passed.
This is consistent with sum -s, cksum, and sum implementations
on other platforms.  This should not cause significant compat
issues, as multiple fields are already output, and so already
need to be parsed.

* src/sum.c (bsd_sum_file): Output the file name
if any name parameter is passed.
* tests/misc/sum.pl: Adjust accordingly.
* doc/coreutils.texi (sum invocation): Likewise.
* NEWS: Mention the change in behavior.