Apparently the su(1) program in the fileutils package cannot handle
multiple-length -c arguments.  (The -c argument is used by su(1) to execute
a program or script other than a shell as a different user.)

For example, I cannot get this to work, although the man page says it
should, and it works correctly in other su(1) implementations:

su user -c /path/to/exec argument argument ...

I've also tried this form:

su -c "/path/to/exec argument argument" user

and it doesn't work either.