bug#81541: Script-breaking changes to "sum" command

At some point between 8.3 and 9.5, the behavior of the sum command changed.

It used to not print the filename, when only one file was checked.
Now it always prints the filename no matter what.

Scripts which compare checksums of individual files are now broken because
they don't expect to see a filename there, and therefore the checksums of
two different files no longer match.

This is breaking scripts.  It should have been added as an option for the
future to preserve script behavior, but, failing that, at least an option
should have been added to make for a trivial fix for scripts.