pre-commit triggers

Attached is a patch to provide a new event trigger that will fire on
transaction commit. I have tried to make certain that it fires at a
sufficiently early stage in the commit process that some of the evils
mentioned in previous discussions on this topic aren't relevant.

The triggers don't fire if there is no real XID, so only actual data
changes should cause the trigger to fire. They also don't fire in single
user mode, so that if you do something stupid like create a trigger that
unconditionally raises an error you have a way to recover.

This is intended to be somewhat similar to the same feature in the
Firebird database, and the initial demand came from a client migrating
from that system to Postgres.

cheers

andrew