psql tab completion for user functions and if explicitly required also "pg_"

Hello,

I think it would be nice to have tab completion for user functions after
`SELECT`, and also for `pg_` in some cases. There was already a
discussion around this here
/messages/by-id/CACLU5mQBYaMgGzJ-ZoG4MrBuXLsREcQbiY4pqsnYjMaMQP6hQg@mail.gmail.com%C2%A0,
however as that discussion is ~ 1 year old I thought it's better to
start a new one.

Attaching a patch, keeping in mind some doubts that were expressed in
the old discussion.

Cheers,

Florin Irion
https://www.enterprisedb.com