sum: does not always display filename

[Matt Weinstein]

Problem:
sum does not output the filename of the file being sum(med) if only one
filename is supplied.

This could lead to problems with e.g. archiving systems, etc.

This is inconsistent and undocumented to the best of my knowledge.

Repeat by:
    echo "hi there" > foo
    echo "hi there" > bar
    $ sum foo
    31345     1
    $ sum foo bar
    31345     1 foo
    31345     1 bar
Note missing "foo" in first sequence above.

Workaround:
None found.