ipn: declare NotifyWatchOpt consts without using iota

Updates #cleanup
Updates #1909 (noticed while working on that)

Change-Id: I505001e5294287ad2a937b4db61d9e67de70fa14
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f