ipn/ipnlocal: ensure Persist information is saved to server mode start state

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>

ipn/ipnlocal/local.go

@@ -861,6 +861,7 @@ func (b *LocalBackend) setClientStatus(st controlclient.Status) {
 		b.setNetMapLocked(st.NetMap)
 		b.updateFilterLocked(st.NetMap, b.prefs)
 	}
+	userID := b.userID
 	b.mu.Unlock()
 
 	// Now complete the lock-free parts of what we started while locked.
@@ -870,6 +871,8 @@ func (b *LocalBackend) setClientStatus(st controlclient.Status) {
 				b.logf("Failed to save new controlclient state: %v", err)
 			}
 		}
+		b.writeServerModeStartState(userID, prefs.View())
+
 		p := prefs.View()
 		b.send(ipn.Notify{Prefs: &p})
 	}