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>