Tab completion: don't offer valid constraints in VALIDATE CONSTRAINT.
Improve psql so that "ALTER TABLE foo VALIDATE CONSTRAINT <TAB>"
only offers not-convalidated entries. While it's not formally
wrong to offer validated ones, there's not much point either,
and it can save some typing if we incorporate this knowledge.
David Fetter, reviewed by Aleksander Alekseev
Discussion: https://postgr.es/m/
20210427002433.GB17834@fetter.org