Then use it in tailcfg which had it duplicated a couple times.
I think we have it a few other places too.
And use slices.Equal in wgengine/router too. (found while looking for callers)
Updates #cleanup
Change-Id: If5350eee9b3ef071882a3db29a305081e4cd9d23
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>