cmd,ipn/ipnlocal,tailcfg: implement TKA disablement

* Plumb disablement values through some of the internals of TKA enablement.
 * Transmit the node's TKA hash at the end of sync so the control plane understands each node's head.
 * Implement /machine/tka/disable RPC to actuate disablement on the control plane.

There is a partner PR for the control server I'll send shortly.

Signed-off-by: Tom DNetto <tom@tailscale.com>

ipn/ipnlocal/state_test.go

@@ -248,6 +248,10 @@ func (cc *mockControl) SetNetInfo(ni *tailcfg.NetInfo) {
 	cc.called("SetNetInfo")
 }
 
+func (cc *mockControl) SetTKAHead(head string) {
+	cc.logf("SetTKAHead: %s", head)
+}
+
 func (cc *mockControl) UpdateEndpoints(endpoints []tailcfg.Endpoint) {
 	// validate endpoint information here?
 	cc.logf("UpdateEndpoints:  ep=%v", endpoints)