psql: Improve CREATE INDEX CONCURRENTLY tab completion

The completion of CREATE INDEX CONCURRENTLY was lacking in several ways
compared to a plain CREATE INDEX command:

- CREATE INDEX <name> ON completes table names, but didn't with
  CONCURRENTLY.

- CREATE INDEX completes ON and existing index names, but with
  CONCURRENTLY it only completed ON.

- CREATE INDEX <name> completes ON, but didn't with CONCURRENTLY.

These are now all fixed.