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 
https://www.postgresql.org/message-id/CACLU5mQBYaMgGzJ-ZoG4MrBuXLsREcQbiY4pqsnYjMaMQP6hQg%40mail.gmail.com , 
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