Support new syntax and improve handling of parentheses in psql tab-completion.

Newly supported syntax are:
  - ALTER {TABLE|INDEX|TABLESPACE} {SET|RESET} with options
  - ALTER TABLE ALTER COLUMN {SET|RESET} with options
  - ALTER TABLE ALTER COLUMN SET STORAGE
  - CREATE INDEX CONCURRENTLY
  - CREATE INDEX ON (without name)
  - CREATE INDEX ... USING with pg_am.amname instead of hard-corded names
  - CREATE TRIGGER with events
  - DROP AGGREGATE function with arguments