Tab completion of function arguments not working in all cases

Hi,

I noticed this while testing 9.2, but it seems to go back to at least
8.3. Tab completion of function arguments doesn't work if the function
is schema-qualified or double-quoted. So for example,

  DROP FUNCTION my_function ( <TAB>

completes the functions arguments, but

  DROP FUNCTION my_schema.my_function ( <TAB>

doesn't offer any completions, and nor does

  DROP FUNCTION "my function" ( <TAB>


The attached patch fixes these problems by introducing a new macro
COMPLETE_WITH_ARG, similar to the existing COMPLETE_WITH_ATTR, which
seems to be the nearest analogous code that covers all the edge cases.

Regards,
Dean