Numerous issues have been filed concerning an inability to install and run
Tailscale headlessly in unattended mode, particularly after rebooting. The
server mode `Prefs` stored in `server-state.conf` were not being updated with
`Persist` state once the node had been succesfully logged in.
Users have been working around this by finagling with the GUI to make it force
a state rewrite. This patch makes that unnecessary by ensuring the required
server mode state is updated when prefs are updated by the control client.
Fixes https://github.com/tailscale/tailscale/issues/3186
Signed-off-by: Aaron Klotz <aaron@tailscale.com>
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