Windows: Add shutdown event log message

Windows: Add shutdown event log message

Add an explicit message that the shutdown sequence is complete.
This is necessary because during a Windows OS shutdown, the service
is frequently killed prior to the memory mapped file is fully released.

Reviewed-on: http://gerrit.openafs.org/5053
Reviewed-by: Derrick Brashear <shadow@dementia.org>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>
(cherry picked from e91f16ebb704a431f9731bb39bf9b5621b7f6ad0)

Change-Id: I80685fae2b581171c580b039ae029ca69bea467c
Reviewed-on: http://gerrit.openafs.org/5132
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>