tailcfg, control/controlclient: make Debug settings sticky in a map session [capver 37]

Fixes #4843

Change-Id: I3accfd91be474ac745cb47f5d6e866c37d5c5d2d
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f