Adjust psql's new \ef command to present an empty CREATE FUNCTION template

Adjust psql's new \ef command to present an empty CREATE FUNCTION template
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.)