psql tab-completion for new syntax

We've added some new syntax in HEAD, but psql tab-completion is out of sync.
Here is a patch to support the following syntax in tab-completion:

    - top-level DO
    - ALTER TABLE/INDEX/TABLESPACE SET/RESET with options
    - ALTER TABLE ALTER COLUMN SET/RESET with options
    - CREATE TRIGGER with events

The fix is not a stopper to alpha release, but I'd like to
add it before beta release. Comments welcome.

Regards,
---
Takahiro Itagaki
NTT Open Source Software Center