patch for distinguishing PG instances in event log v2

Hello,

The attached file is a revised patch which reflects all review comments by
Magnus in:

http://archives.postgresql.org/pgsql-hackers/2011-07/msg00839.php

I made sure the previous tests (both custom and default "PostgreSQL" event
source) succeeded.

I'm submitting this to the currently open CommitFest 2001-9 shortly. Please
review it again.

Regards
MauMau