psql tab completion enhancements

Hi,

psql's tab completion has the following problem:

If we have the following syntax for example:

SET SESSION AUTHORIZATION <user>;
SET SESSION AUTHORIZATION DEFAULT;

After "SET SESSION AUTHORIZATION", the tab completion can offer a list of
roles or the string constant "DEFAULT". However it can't offer both because
it can't get a list of roles and add a string constant to this list.

The appended patch adds the functionality of lists that can be extended with
constants.

Then you get:

template1=# SET session AUTHORIZATION <tab>
DEFAULT fred joe john

I did proof-of-concept examples to add a constant to a

- list from a query
- list from a schema query
- list of table attributes

Joachim