psql tab completion improvements, from Greg Sabino Mullane:

* Made DELETE into "DELETE FROM"
* Moved ANALZYE to the end of the list to ease EXPLAIN / VACUUM
  conflicts
* Removed the ANALYZE xx semicolon completion: we don't do that anywhere
  else
* Add DECLARE support
* Add parens for DROP AGGREGATE
* Add "CASCADE | RESTRICT" for DROP xx
* Make EXPLAIN <tab> a lot smarter
* GROUP "BY" and ORDER "BY"
* "ISOLATION" becomes "ISOLATION LEVEL"
* Fix error in which REVOKE xx ON yy was receiving "TO", now gets "FROM"
* Add GRANT/REVOKE xx ON yy TO/FROM choices: usernames, GROUP, PUBLIC
* PREPARE xx <tab> AS "SELECT | INSERT | UPDATE | DELETE"
* Add = at end of UPDATE xx SET yy
* Beef up VACUUM stuff