per-statement triggers

This patch implements FOR EACH STATEMENT triggers, per my email to
-hackers a couple days ago.

Notes/caveats:

        - added regression tests for the new functionality, all
          regression tests pass on my machine

        - added pg_dump support

        - updated PL/PgSQL to support per-statement triggers; didn't
          look at the other procedural languages.

        - there's (even) more code duplication in trigger.c than there
          was previously. Any suggestions on how to refactor the
          ExecXXXTriggers() functions to reuse more code would be
          welcome -- I took a brief look at it, but couldn't see an
          easy way to do it (there are several subtly-different
          versions of the code in question)

        - updated the documentation. I also took the liberty of
          removing a big chunk of duplicated syntax documentation in
          the Programmer's Guide on triggers, and moving that
          information to the CREATE TRIGGER reference page.

        - I also included some spelling fixes and similar small
          cleanups I noticed while making the changes. If you'd like
          me to split those into a separate patch, let me know.

Cheers,

Neil

--
Neil Conway <neilc@samurai.com> || PGP Key ID: DB3C29FC