pg_autovacuum Win32 service patch #2

[Third attempt to send this - dunno where they're all going!]

I forgot to CC the start of this to -hackers last time - please see
http://archives.postgresql.org/pgsql-hackers-win32/2004-05/msg00034.php
for background.

Following Magnus' suggestions yesterday I made the following changes:

- The event source is now "PostgreSQL Auto Vacuum"
- Error levels have been added to the error handler to standardise log
entry format and allow use of Win32 event types.
- When installing as a windows service the following options can be used
to specify the uid/pwd to run the service as:

-N <username>
-W <password>

- Other options specified when installing as a service are stored in the
registry for use each startup, INCLUDING DATABASE PASSWORD.

In addition, I've also cleaned up the service startup code so that it
terminates properly if there is an error at startup, and modified
InstallService and RemoveService to register/deregister the service as
an event source.

Any comments/criticisms/gasps of horror at all the win32 code? :-)

Regards, Dave