tailcfg, control/controlclient: add method to exit client from control plane

Change-Id: Ic28ef283ba63396b68fab86bfb0a8ee8f432474c
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>
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Internal Server Error - Gitea: Git with a cup of tea
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Gitea Version: 1.28.0+dev-477-g8b6ad49a5f