patch for distinguishing PG instances in event log

Hello,

I wrote and attached a patch for the TODO item below (which I proposed).

Allow multiple Postgres clusters running on the same machine to distinguish
themselves in the event log
http://archives.postgresql.org/pgsql-hackers/2011-03/msg01297.php
http://archives.postgresql.org/pgsql-hackers/2011-05/msg00574.php

I changed two things from the original proposal.

1. regsvr32.exe needs /n when you specify event source
I described the reason in src/bin/pgevent/pgevent.c.

2. I moved the article for event log registration to more suitable place
The traditional place and what I originally proposed were not best, because
those who don't build from source won't read those places.

I successfully tested event log registration/unregistration, event logging
with/without event_source parameter, and SHOWing event_source parameter with
psql on Windows Vista (32-bit). I would appreciate if someone could test it
on 64-bit Windows who has the 64-bit environment.

I'll add this patch to the first CommitFest of 9.2. Thank you in advance for
reviewing it.

Regards
MauMau