Windows: Backup and restore configuration across installs

Windows: Backup and restore configuration across installs

The MSI installer for OpenAFS does not preserve configuration data
across installs.  This patch creates a backup of specific
configuration registry values when uninstalling OpenAFS and uses this
backup when subsequently installing OpenAFS.

Change-Id: Ie30f8f7f0eada3f0aeef2341b04aca55657ec864
Reviewed-on: http://gerrit.openafs.org/2977
Tested-by: BuildBot <buildbot@rampaginggeek.com>
Reviewed-by: Jeffrey Altman <jaltman@openafs.org>
Tested-by: Jeffrey Altman <jaltman@openafs.org>