for editing if no function name is specified.  This seems a much cleaner way
to offer that functionality than the original patch had.  In passing,
de-clutter the error displays that are given for a bogus function-name
argument, and standardize on "$function$" as the default delimiter for the
function body.  (The original coding would use the shortest possible
dollar-quote delimiter, which seems to create unnecessarily high risk of
later conflicts with the user-modified function body.)