Omit the persist argument to opentap to leave the persistence flag unchanged

Previously when terminating a unikernel process that was using a persistent tapX device for Tcpip_stack_direct, the device was deleted.

With the next release of tuntap, this change will allow the tap device to persist after terminating a unikernel process.

With the current tuntap v1.0.0, this change won't make any difference because ~persist:false is the default anyway.