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(at)samurai(dot)com> || PGP Key ID: DB3C29FC