tun: guard Device.Events() against chan writes

Signed-off-by: Jordan Whited <jordan@tailscale.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

tun/tun.go

@@ -24,6 +24,6 @@ type Device interface {
 	Flush() error                   // flush all previous writes to the device
 	MTU() (int, error)              // returns the MTU of the device
 	Name() (string, error)          // fetches and returns the current name
-	Events() chan Event             // returns a constant channel of events related to the device
+	Events() <-chan Event           // returns a constant channel of events related to the device
 	Close() error                   // stops the device and closes the event channel
 }