Add more tab completion support for ALTER DEFAULT PRIVILEGES in psql.

Add more tab completion support for ALTER DEFAULT PRIVILEGES in psql.

This adds tab completion of "GRANT" and "REVOKE [GRANT OPTION FOR]"
for ALTER DEFAULT PRIVILEGES, and adds "WITH GRANT OPTION" for
ALTER DEFAULT PRIVILEGES ... GRANT ... TO role.

Author: Vignesh C, with cosmetic adjustments by me
Reviewed-by: Shubham Khanna, Masahiko Sawada
Discussion: https://postgr.es/m/CALDaNm1aEdJb-QJi%3DGWStkfj_%2BEDUK_VtDkn%2BTjQ2z7HyU0MBw%40mail.gmail.com