Some improvements for the tab-completion of psql. This should
address all of the items in the todo list and adds some new
things as well. Specifically:
* Add support for ALTER SEQUENCE ...
* Add "RENAME TO" for ALTER TRIGGER xx ON yy
* Pick proper table for ALTER TRIGGER xx ON ...
* Support for ALTER USER xxx ...
* Fix ALTER GROUP xxx DROP ...
* Fix ALTER DOMAIN xxx DROP ...
* Remove "OWNER TO" from ALTER DOMAIN xx DROP ...
* Fix ALTER DOMAIN xx SET DEFAULT ..
* Prevent ALTER INDEX xxx SET TABLESPACE from using "TO"
* Support for ALTER LANGUAGE xxx (RENAME TO)
* More support for ALTER TABLE xxx ALTER COLUMN xxx ...
* More support for COPY