LocalBackend transitions to ipn.NoState when switching to a different (or new) profile.
When this happens, we should unconfigure wgengine to clear routes, DNS configuration,
firewall rules that block all traffic except to the exit node, etc.
In this PR, we update (*LocalBackend).enterStateLockedOnEntry to do just that.
Fixes#15316
Updates tailscale/corp#23967
Signed-off-by: Nick Khyl <nickk@tailscale.com>