cmd/tailscale/cli,ipn/ipnlocal: restrict logout when AlwaysOn mode is enabled

In this PR, we start passing a LocalAPI actor to (*LocalBackend).Logout to make it subject
to the same access check as disconnects made via tailscale down or the GUI.

We then update the CLI to allow `tailscale logout` to accept a reason, similar to `tailscale down`.

Updates tailscale/corp#26249

Signed-off-by: Nick Khyl <nickk@tailscale.com>
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