util/osdiag, util/winutil: expose Windows policy key

The Windows base registry key is already exported but the policy key was
not. util/osdiag currently replicates the string rather than the
preferred approach of reusing the constant.

Updates #cleanup

Change-Id: I6c1c45337896c744059b85643da2364fb3f232f2
Signed-off-by: Adrian Dewhurst <adrian@tailscale.com>
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