Improving psql autocompletion for SET LOCAL / SET SESSION

Hi all,

I have written a small patch to improve psql autocompletion for SET LOCAL /
SET SESSION, which did not show any autocompletion options before. The
patch implements the autocompletion changes to show the standard list of
set vars, plus the special options TIME ZONE, SCHEMA, and NAMES.

Thanks,
Álvaro Rodríguez