In this PR, we move the syspolicy.FlushDNSOnSessionUnlock check from service startup
to when a session change notification is received. This ensures that the most recent policy
setting value is used if it has changed since the service started.
We also plan to handle session change notifications for unrelated reasons
and need to decouple notification subscriptions from DNS anyway.
Updates #12687
Updates tailscale/corp#18342
Signed-off-by: Nick Khyl <nickk@tailscale.com>