ipn/ipnlocal: fix deadlock in resetControlClientLocked

resetControlClientLocked is called while b.mu was held and
would call cc.Shutdown which would wait for the observer queue
to drain.
However, there may be active callbacks from cc already waiting for
b.mu resulting in a deadlock.

This makes it so that resetControlClientLocked does not call
Shutdown, and instead just returns the value.
It also makes it so that any status received from previous cc
are ignored.

Updates tailscale/corp#12827

Signed-off-by: Maisem Ali <maisem@tailscale.com>

control/controlclient/client.go

@@ -25,6 +25,9 @@ const (
 // Client represents a client connection to the control server.
 // Currently this is done through a pair of polling https requests in
 // the Auto client, but that might change eventually.
+//
+// The Client must be comparable as it is used by the Observer to detect stale
+// clients.
 type Client interface {
 	// Shutdown closes this session, which should not be used any further
 	// afterwards.