[client] Scope the iOS file drop listener to the tunnel interface

The receiver's host listener is a socket of the Network Extension, so the
SYN-ACK of an accepted connection followed the extension's own-traffic
bypass onto the physical interface and the sender never got an answer.
Bind the listeners to the tunnel interface index the same way the dial
already is; accepted sockets inherit the scope. Other platforms pass a
nil control and keep the current behavior.

client/internal/engine_filedrop_dial.go

@@ -12,3 +12,7 @@ func fileDropOSDial(WGIface) filedrop.DialFunc {
 	dialer := &net.Dialer{}
 	return dialer.DialContext
 }
+
+func fileDropListenControl(WGIface) filedrop.ListenControl {
+	return nil
+}