Suggestion from Arthur:

> Would it make sense, or even be possible, to make a procedure that returns the symbol command or script depending on which it is?

A program can be both a command and a script at the same time. Therefore
it would be simpler to have two predicates: `command?` and `script?`. I
left them out since testing for a #f return value from `command-name`,
`script-file`, or `script-directory` accomplishes the same goal.

`command` and `script` are also such common words that it would be nice
to reserve those procedures for users. For example, if a Scheme program
has its own command datatype, it's nice if `command?` can refer to that.
Then again, I guess that would be nice with `command-name` as well.

Currently one can write code like this:

     (define csv-file
       (or (and (script-directory)
                (string-append (script-directory) "data.csv"))
           (get-environment-variable "CSV_FILE")
           (error "Cannot find CSV file")))

We can add the explicit predicates if there is demand.